Rock (Tool, Porcupine Tree, Pink Floyd, Rage, Muse) Post-Rock (Explosions in the Sky, Mono, Pelican, Mogwai) Hip-Hop (A Tribe Called Quest, Gang Starr, Outkast, Digable Planets, The Coup)
The post-rock/instrumental stuff probably works the best since you aren't tempted to sing, and it helps to listen to something that is a little more repetitive.
I also like to listen to some down-tempo/electronic stuff like Thievery Corporation, Kruder & Dorfmeister and Sound Tribe Sector 9.
i'm probably one of the rare developers who listens to a lot of death metal though. the mathematical structure and intensity helps me get into that "flow state".
a really nice set of studio quality headphones is one of those things that i consider a professional necessity.