Leatherworkers, blacksmiths, engineers, enchanters, tailors and even alchemists in a way can sell their "ability to craft" something for a small fee. The fee you can ask will generally depend on the rarity of the item, and how much effort or cash you put into raising your skill. Don't craft anything for free, unless something is ridiculously easy to make or you're making the item for friends. A small tip of 1-30g should be quite reasonable in most cases. Time is money after all.
Another way to get cash by crafting, is to provide basic materials. Have your customer bring the rare ingredients (primal might or nethers for example) while you bring cloth and other primals. This way you can charge a good price for the bulk of the materials and still seem like a good person by not asking for a tip.
Most useful engineering recipes require the user to have engineering skill. Because of this you'll find it's hard to sell anything that's BoE.
Scopes, Mana and health potion injectors are the main things I've been able to craft for others. But the gains are small. Other than for friends or inside the guild, there just isn't a great lot of market an engineer can use. One should probably blame Blizzard in part, if the potion injectors had been useable by everyone from the start there might have been more people wanting to use them still. Even the arrow makers are cool, but at this point in time they simply refuse to sell because blizzard also added bullets and arrows from factions at the same time. Jewelcrafters really did get most of the love in TBC.
As an Enchanter, your main income is by selling enchants, much to the annoyance of the people in the general trade channel. But screw them, you need cash as much as they do. Just because they can put their stuff on the AH while you can't doesn't mean you don't deserve a quick buck right? Also, Blizzard promised they'd allow enchanters to put some of the more basic enchants on the AH when WotLK is released. The Mongoose enchant scroll for example, and many more. See wow insider for more info on that.
Unlike popular belief, enchanting is pretty easy to raise, and not at all that expensive either. It is time consuming though. You should perform "free" enchants whenever able to get your skill maxed out as soon as possible. The real money is up at the top, and especially a stealth class like a druid or rogue will be able to get good money's worth out of this skill by disenchanting stealth run drops. Having an alt with Tailoring or Leatherworking will also be a great boon to obtaining the materials you need. Once your skill reaches about 340 you can make Superior Wizard Oil all the way to 360 and sell the results on the auction house or trade them with guildies for getting more materials as you work. After that, find some green bind on pickup recipes and perform those for "free" (no tip, their mats) in Shattrath. You'll be 375 in no time.
Once you manage to get a hold of some enchants like dexterity on boots, sunfire, soulfrost, mongoose, executioner and spellsurge you should be able to get enchanting tips varying between 10-50g per craft. It's a slow process, but there will always be customers aplenty. Especially when a new PvP season starts. Be sure you always have some enchanting materials on the AH, as potential customers will run off to check the most recent prices as soon as you spam the trade channel. Just make sure you use an alt to post the items.
Void shattering is currently a reasonable gain, though this will vary per server. Voids going for 30-40g, while large prismatic shards will sell for 20-30g. Expect lots of competition. Also a thing to keep in mind is that the price of void crystals might skyrocket once the new BoP WotLK wands become available.
Any useless quest rewards can be disenchanted instead of selling it to the vendors. Especially late level quests will often give you blue items which can be made into shards. Gear upgrades won't go to a loss either, just shard your old gear instead of vendoring it.
You got several options as a tailor, but none that'll really make you money quick and dirty. More like clean and slow. You can take spellcloth, shadowcloth or mooncloth tailoring when you reach a high enough level. Mooncloth is a good way to get a lot of bags produced.
Mind, these prices are for my server, your will likely be somewhat different.
If there's a market on your server for a particular type of cloth, and speccing for that school benefits your class (mage, priest, lock, etc) by all means go for it. I've made my warlock alt a mooncloth vendor however, both because primal water and primal life are ridiculously cheap and easy to get, and to make a constant stream of mooncloth bags for my other characters to use. Individual bags sell for about ~450g on my server, so you'd generally be better off selling the individual cloth. That is, on my server at least.
Much like enchanting, if you can manage to get some rare recipes you'll be able to craft items for others with a decent tip. Belts from SSC/TK, bracers from BT/HY and gloves from Sunwell are good money makers with a decent level of demand. Items like spellstrike hood or pants will also work decently, but it's generally not hard for a player to find more than one tailor able to craft those.
On top of that you'll be able to make Runic Spellthread or Golden Spellthread depending whether you ally yourself with Scryers or Aldor. These two enchants are a good way to use your badges and turn them into something profitable.
Of course, there's always epic gems you can buy for them as well.
See tailoring and enchanting regarding rare patterns and tips. The same applies to Leatherworking. On top of that LW is one of the best two professions to have as a raider currently, so it's certainly worth investing in.
Leatherworkers can also make Nethercobra Leg Armor and Nethercleft Leg Armor. If you also have an alt capable of skinning then you'll be able to get Thick Clefthoof Leather and Cobra Scales quite easily and for good profit be able to sell leg armor kits.
See tailoring and enchanting regarding rare patterns and tips. The same applies to Blacksmithing. Other than that, there just isn't really anything for a blacksmith to make cash on. I could be wrong though, it doesn't happen a lot, but it does happen from time to time. ;)
Transmutation is the magic word. Well, one of the words anyway. Once you are high enough in skill, you can do the quest to become a master of elixirs (it's the cheapest way trust me), and then head to netherstorm to pay 150g for becoming a transmutation master. This way will save you having to make 5 Primal Mights for that greedy ethereal bastard.
Once that's completed you can start producing a Earthstorm diamonds once every 20 hours. Sometimes you get lucky and get 2-5 as a bonus cash cow. Hand these to a jewelcrafting friend or alt and make Relentless Earthstorm Diamond. These sell for about ~200-250g where I come from. This provides you with a ridiculously good turnover value for just 2 Primal Earth, 2 Primal Water and some green gems. Sadly, you can only transmute once every 20 hours.
If slow and easy cash isn't your thing and you have a crazy herbing alt with a lot of time on his hands hands then you could consider sticking to elixir mastery or doing the potion mastery quest instead. If you're a frequent user of potions for raids, or have a goldmine going on at the AH because some mana hungry healers or mages are constantly buying out your stocks, then you may want to considder your other options. (Also see the Gathering - Herbalism section further down) For potion mastery this would mean you'll be making mostly Super Mana potions. These sell often and still rake in a good price even on the bad days.
Elixir mastery is able to get procs a lot more often than transmutation mastery, and a lot of raid buffs are elixir based so you're almost guaranteed to have customers. Getting an average of 10-30% extra elixirs such as when making a few stacks of the major agility one will certainly not hurt your bank funds. If you got a cheap source for Fel lotus, you could even make flasks and get a chance to double proc on those.
The details to inscription are still mostly unknown at this time. It'll use herbs to produce inks, and create inscriptions useable as temporary enchants on chests, or permanent enchants on spells. How much of this can be done by another scribe, or whether there will be a lot of inscriptions only available to the scribe remains to be seen. WotLK Wiki has a bit more info than the other sites about this profession at the moment.
If you can manage to raise your inscription as one of the first few on the server, there's no question there will be a lot of cash to be made however. Much like Jewelcrafting in the early days.
Gathering is not for everyone, but even the most epic and rich players out there will have done his or her share of work out in the fields. Gathering materials for a certain profession. Hasn't everyone had a character at one point who had skinning, herbalism or mining? Surely yes?
Preparation is key, as it is with everything. What do you intend to gather, how much, how long, where is the least competition, what's your gain per hour, what addons can help speed this up, isn't the auction house flooded with this stuff yet? You get the idea.
You don't "need" a +280% flying mount, but in order to be competitive it's highly recommended for all gathering professions. If you don't have that, and are stuck with a +100% land speed mount, stay away from netherstorm, blade's edge and nagrand. Those zones are not friendly to land based gatherers.
But, first things first. You're going to need a few addons to do this right. Well, "need" is a big word but these addons will improve your efficiency by at least 50%. If you don't have ace2 yet, look here.
Once you got that working, you'll need :
You could be one of the people using cartographer or gatherer, but personally I prefer the minimalist approach. These two addons are the main thing you need to get rich with gathering and will give you a definite edge over any competition who isn't using them.
Engineers need a Mote extractor as well as a pair of goggles or whichever is relevant for your class before they can start gathering. See this list to find out which one you need.
Once you have that, simply mount up and start running around the nagrand zone you like. That's right, this only works in outland for now. Possibly in WotLK as well, but we can't be sure of that yet.
You'll notice my path also hits the fishing pools for "pure water". Primal water isn't worth a "huge lot", but it's still worth taking along your fishing rod just in case. If you're not interested in those, you can simply skip over the path north of Garadar and continue your round. Also keep in mind that quite silly, there's a lot of gas clouds on top of those sky islands which tend to bug out and mess up your maximum profit per hour by a fair margin.
Zangarmarsh is great, usually not as ridiculously populated as Nagrand, and Primal water is generally not all that much worth less than primal air. On average you'll find yourself able to get 4-5 more primal water in the same time that you can farm primal air.
With the opening of Black Temple and Mt.Hyjal there's a lot of guilds in need of Primal shadow. Since these no longer drop from demons an engineer can really rake in some decent cash with this path.
My Netherstorm path sucks, I know. Netherstorm is a horrible place to farm primal mana at it seems, as I've never really managed to get a "lot" of them, so it's quickly discouraging to continue farming them this way.
Mining can be done in pretty much any zone, if you're after adamantite however, I recommend Nagrand, Shadowmoon valley or Netherstorm. You can't really farm eternium or khorium, so just farm rich adamantite as much as you can.
Fel iron ore can best be obtained in Hellfire peninsula. There tends to be average competition there though so you may want to consider flying circles around zangarmarsh if you're in dire need for some green.
You can run around and herb what you or your guildies need, or pick plants that sell good and sell those in stacks at the AH. Here's some commonly used potions and items requiring herbs below.
Flasks other than the Pure death one are fairly underused because of the Marks, which tend to be a lot cheaper.
So which herbs should you be going after? Well, all of them obviously, but primarily the ones worth the most on the auction house. Such as Terocone and nightmare vine.
There are two main things you'll want to farm as a skinner. As mentioned earlier this is because of the leg armor kits.
You might not think it, but yes even fishing can be used to make some money. What few people seem to realize however, is that you need to fish "all" pools in order to get the Pure water pools to spawn more frequently. And not just fly over once an hour in the hope for a pool to spawn. That may work with the pools in Nagrand, but there's possible spawn locations in zangarmarsh and Terrokar forest as well. Fish all pools.
There's a fishing daily quests you can do as well, but in the grand scale of things that's such a small amount of cash you gain. It's hardly worth mentioning here.
And last but not least Deviate fish. On a pure time = money scale these score pretty low, 50s per fish but still, if I mentioned the daily fishing quest, these can't be left out. Especially since certain guild members eat about a dozen of these per night.
In the end
If you find any serious typos or got suggestions, just leave a message after the *beep*.
ps: I ate cowgirl's bacon sandwich.