Well like most people already made a note of on here, it depends on what part of the game you're working on. I like to try and set the mood for area by listening to something that will match the sort of atmosphere. Me I mainly listen to metal, but I don't limit myself to just that. I like to listen to a bunch of different music. I quite like a lot of classical stuff or something more influenced by classical music, same goes with like jazz and funk. A lot of old 80's rock, maybe even some abstract stuff here and there. Not big into the electronic/techno, rap, or country, but that's just personal taste. Also, game soundtracks from games that I really enjoyed as well as the music to go with it is real nice too. If i'm stuck on something and need to get my brain jogging I like to listen to some progressive or technical stuff. And from there I can usually find something within my library of tunes a little more mellow or aggressive while still staying in the technical/progressive genres.
For some mellow (but still has some bite to it) I might listen to something like; Art By Numbers, some of The HAARP Machine, some Corelia, Muse, Coheed And Cambria, Evan Brewer, Mendel, Ninjaspy, Tool.
Some of them have some mellow songs along with more heavier songs so, for more aggressiveness I'd probably turn to stuff like; Obscura, The Zenith Passage, All Shall Perish, Beyond Creation, Derelict, Divinity, some Job For A Cowboy, The Faceless, Revocation or what have you
This kinda stuff will get me inspired. Some people would probably need different styles of music to fit their needs, this is just my 2 cents, whatever works for you guys