Leatherworking

Allakhazam's Leatherworking page is here, and Thottbott's is here. Thottbot's is a little better, IMO, but if you're interested in drop- or vendor-only patterns, Allakhazam's page lists these on their page, just click the <Pattern:X> link under the name of the item on the list above to see where to get it.

For Alliance, the Artisan Leatherworker is in Darnassus, and the Master Leatherworker is in Aerie Peak.

For Horde, the Artisan Leatherworker is in Thunder Bluff, and the Master Leatherworker is in Camp Mojache.

After level 40, and when your leatherworking skill is sufficiently high (Artisan is required), you can specialize in one of the 3 branches of leatherworking:

Tribal Leatherworking quest Items generally boost intellect and spirit, good for druids. There is a Tribal Leatherworker south of Grom'gol.

Elemental Leatherworking quest Items generally boost agility and stamina, good for rogues and possibly hunters. There is an Elemental Leatherworker west of Thoradin's Wall, on the Arathi side.

Dragonscale Leatherworking quest Makes mail, good for hunters possibly, since they can train to wear mail at lvl 40.