Harbinger Mercenaries

You'll find a lot of meat when fighting creatures, and you can cook that to make edible food. You eat it to regen health. Food cooked by players also gives the Well Fed buff, which gives you bonus sta and spi for 15 minutes. Rogues at one point do a quest for which they're rewarded the recipe for Thistle Tea (instantly restores 100 energy), which requires Swiftthislte (obtained from herbalism [I buy mine from the Auction House]). Recommended for rogues, warlocks, and warriors.

Gathered materials used: Meat (obtained from killing animals).

Almost required if you're a class who can't heal, but you want to be able to solo. You turn cloth (the same kind used by Tailors) in to bandages, and heal yourself. Bandaging is a channeled skill, so when bandaging you can't move, and if anyone hits you, or the person being healed, it stops. Though you can bandage in combat if neither the healer nor the healed is being hit. A bandaged player cannot be bandaged again for 60 seconds after the beginning of bandaging. Recommended for hunters, rogues, warlocks, and warriors. Priests and pallys may use them too to reduce the mana needed to heal after a battle.

Gathered materials used: Cloth (obtained from killing humanoids).

You catch fish. They can be eaten, fed to pets, cooked with Cooking, or sold to vendors (due to commonality, fish sell for less than other foods). Recipes for cooking fish are easily found at fishing or cooking suppliers. You also sometimes find other cool loot in the water(some of which is nigh impossible find any other way), but mostly fish. Recommended for people who like to fish, want something to help increase their cooking fast, or hunters who need to feed their pets.

Lets you make healing potions, mana potions, protection potions (absorbs damage of a certain type for a period of time), rage potions for warriors, speed potions, breathing potions, a bunch of different buff potions... Lots of helpful things here. Priests who bring their party members potions get mad loving. Though anyone can get use out of Alchemy, I just mention priests because they're already the definition of a character geared towards helping their party, and so alchemy is a popular choice with them.

Gathered materials used: Herbs (obtained from herbalism).

Not as good on low levels, but can make some pretty badass stuff on high levels. Makes weapons and armor (chain mail until level 40, and then plate armor).

Gathered materials used: Ore, stone, gems (all obtained from mining), cloth (obtained from killing humanoids).

Specializations: Weaponsmithing, Armorsmithing. Weaponsmithing further specializes in to Axesmithing, Hammersmithing or Swordsmithing.

On Blizzard's site this is actually listed as a service profession (along with First Aid) rather than a crafting profession. This is because you don't make things, you apply bonuses to them. Enchanters get materials by disenchanting items which are green (Uncommong), blue (Rare), or purple (Epic, which would be foolish to disenchant [unless its bound to you and you can't use it], because these are worth fortunes). Shards typically come from more valuable items (blue), so enchanters like to go to instances a lot to get rare BoP (Bind on Pickup) items to disenchant.

Enchanting is extremely expensive. If you choose this profession and don't have help with it you will be broke. And by that I mean my broke lvl 30 rogue leatherworker made my friend's lvl 50 warlock enchanter look poor. This is because you keep destroying uncommon or better equipment, which is normally sold in the auction house. And so, I strongly recommend against enchanting on any character that has to pay for their mount at 40. A mount is a huge expense (especially for the epic mount at level 60 which costs an obscene 1000 gold), and if you enchant, you basically cannot afford it at all. So Paladins and Warlocks make the most sense for enchanters in this regard. But with help from others, or if you have a higher level character, you can make anyone an enchanter.

Tailoring goes well with enchanting because you can make items and then disenchant them. Although you won't get the better materials, like shards, it's still very helpful. Also, being able to make bags rocks.

So I'd say Warlocks make the most sense for an enchanter, since they can also use Tailoring to good effect, and don't have to pay for a mount. Paladins work well too, though Tailoring seems a bit stranger for them. Still works though, you'll just get funny looks from people passing by the tailoring shop seeing you there on your warhorse in your platemail. :P

Gathered materials used: Dust, essences, and shards (all obtained from disenchanting enchanted equipment).

Works for just about anyone. The goggles I like especially, since you can wear them way before you'll find any normal headgear (which is normally around lvl 30), and they give bonuses to stamina and spirit, which are good for anyone. on high levels of engineering you can make some really cool goggles of other kinds, mostly good for caster class (bonus int and spi). The dynamite is also very good. If you have others helping you powerlevel engineering you'll be throwing dynamite way more powerful than your level. My mage can do way more AoE (Area of Effect) damage with her badass dynamite than her AoE mage spells. Engineering also gets a lot of cool little contraptions. Also, gnomes get +15 starting and max engineering, so you'll have an easier time of it as a gnome. But engineering works for pretty much anyone if you want to use it.

Most items are only usable by engineers. Like dynamite, bombs, goblin jumper cables, remotes for controlling mechanical opponents, mechanical helpers and explosive sheep. Yes, explosive sheep. There are still a variety of items usable by anyone though, like some random quest items, EZ-Thro Dynamite, mechanical squirrels, chickens, and dragonlings.

Gathered materials used: Ore, stone, gems (all obtained from mining), cloth (obtained from killing humanoids). A few odds and ends as well, like Fused Wiring for Goblin Jumper Cables, which are often found in Gnomeregan, but people often sell them to vendors thinking they're worthless (engineers often pay 1 gold for Fused Wiring).

Specializations: Gnomish Engineering or Goblin Engineering. Gnomish focuses on gadgets and pets who fight (e.g. Gnomish Death Ray, Gnomish Battle Chicken, World Enlarger), and can have odd effects when they fail. Goblin focuses on blowing things up and get very powerful bombs, dynamite, and explosive contraptions that may also damage you if they fail. The level 300 engineering recipe to make the repair bot (which buys your items and repairs your gear) is usable by any kind of engineer with at least 300 engineering.

Having two gathering skills is a big moneymaker. If you're going to be a gatherer only, take Skinning and either Mining or Herbalism. Don't take both Mining and Herbalism because they both use a radar function, and you can only have one on at a time. I think Mining and Herbalism are the real moneymakers, but skinning is very easy and doesn't require a radar effect. Actually, considering how much more leather you'd find than ore and herbs, it's probably an equivalent moneymaker, as well as the easiest to quickly level up and maintain.

Try to pick gathering skills you'll get use out of. You can send herbs to the guild alchemist for potions (healing, mana, buff potions, protection potions, rage potions [for warriors], speed potions, waterbreathing potions), and ore to the blacksmith for weapons/armor. And of course if you use leather you can send your leather to the leatherworker for armor. Engineers also make things for others, but a lot of their stuff is only usable by engineers (see info on Engineering).

Makes the obvious armor type. Druids, hunters, rogues and shaman wear leather. Hunters and shaman can wear mail at 40 and up, but can still get use out of leatherworking via the Dragonmail specialization (see below).

Gathered materials used: Leather and hides (both obtained from skinning). On high levels, Deeprock Salt and Refined Deeprock Salt (both obtained from killing earth elementals) are required for tanning hides, and dragon scales (also gotten via skinning) are used for Dragonmail leatherworking.

Specializations: Dragonmail, Elemental, and Tribal. Dragonmail is for hunters and shaman (makes mail armor, since hunters and shaman use leather until 40 when they can wear mail. So with this category they can start with leatherworking, but still make themselves good stuff on high levels), Elemental for rogues (stuff with agility, and bonus chance to crit and dodge), and Tribal for druids (Intellect and Spirit stuff).

Makes cloth armor as well as bags (particularly the high level uberbags). And anyone can send you materials, since cloth just drops off of humanoid enemies. You can also buy cloth rather cheaply on the market, since everyone finds it. This is actually my favorite profession, even though none of my chars do it right now. :P I think I'll have to change one of them to this...

Gathered materials used: Cloth (obtained from killing humanoids).