Most of WoW's original soundtrack was written by Jason Hayes, who's worked with Blizzard since Starcraft. His most famous piece for Blizzard is probably "A Call to Arms," which probably still gets your heart pumping. He also wrote a lot of the original game's ambient music, including the Ironforge theme that we hear all... the... time. Here's a nice Q&A with Hayes, and while Hayes now works with other videogame companies (including NCSoft), he's also working on his own album, called Mosaic.
But of course he's not the only composer working for Blizzard. Glen Stafford (director of the Audio Department) and Tracy Bush also have written a lot of the music for WoW, as have Russel Brower, Derek Duke, and Matt Uelmen (the last three composers put together music for the Burning Crusade expansion, and Uelmen has won awards for his work on Diablo and Diablo II). Here's an interview with just Uelmen, and here's an interview with Stafford, Uelmen, and Hayes all together.
Unfortunately, I couldn't see that anyone but Hayes is working on other projects, unless L70ETC gets picked up by a record label-- while the members of that band are shrouded in mystery (besides lead singer Samwise Didier and Blizz Prez Mike Morhaime), I'd be very surprised if the Blizzard music team didn't have a representative on there.