Lindsey Stirling - She's a more recent discovery for me, but I think there's something decidedly cool about how she mixes classical violin with modern dubstep beats. She seems to really understand the visual and social mature of modern music interests, too.
The Doors - A long standing favorite of mine. There's a general sense I have that there are always multiple layers in a Doors song that can be unlocked by new experiences or a different perspective. Jim Morrison's voice is probably my favorite of all singers I've heard.
Ra - Ra is just different. Their music has bits of Indian styling, along with Egyptian, built on hard rick sensibilities. On a Ra CD, a song about telling mysoginists to back off can play right after a song about understanding the workings of the universe and make total sense.
Disturbed - I've enjoyed Disturbed for a long time. Their music really speaks to me and singing along to it is generally a sure-fore way for me to relieve stress. They were also pretty entertaining when I've seen them in concert.
Godsmack - I think Godsmack's mix of hard rock and spiritual themes is awesome. They were also excellent in concert when I've been, with lots of energy and fan interaction.
Russel Brower - Composer for a number of Blizzard games and a former sound Imagineer at Disney. Brower is behind a lot of the environmental music I've enjoyed in my life and his discussion of his creative process holds a lot of value even for non-musicians.
Howard Shore - Composer for a number of big movies, including the Lord of the Rings series. I love the way in which the theme music from each Lord of the Rings movie evolves into the next movie's theme. That really helps, I think, show the development of the characters and events of the movies and adds to my immersion.
Elite Tauren Chieftain - They're Blizzard staff, but I look eagerly forward to each new song or Blizzcon performance of these guys. Terran Up The Night has a regular spot in my playlists. Just plain good geeky fun music in the metal vein.
Darkest of the Hillside Thickets - A great punk band hiding in the guise of a Cthulhu-themed group. Their songs all sound fun and the beats are uptempo, but the lyrics pack a plethora of subtle disquieting bits, which is a great match for their inspirational material.
There are definitely more groups than this that I frequently listen to (honorable mentions to They Might Be Giants, Metallica, Journey, Joan Jett, and AC/DC!), but these are the folks I listen to most when brainstorming or writing. They bring out the words that let me create my favorite characters, which is why I'm compelled to share their names.