Also, depends slightly on the subject matter. For mathematics I'd suggest something crazy grimy, e.g. Justice's Planisphere, on the other hand, I find that chill instrumentals, like Explosions in the Sky or something, is really good for more literary pursuits.
at least for me...if it has lyrics, i'll find myself singing a long and not studying.. radiohead and sigur ros have vocals.. but who sings along with them??? haha
Anything involving math or brute for science I go hardcore punk rock (my populations dynamics teacher this semester followed punk bands for five years and wore a Minor Threat shirt first day to class /claim):
-Minor Threat
-Dead Kennedys
-Black Flag
-Misfits
-Bad Religion (lead singer teaches at my college and said hi to me /claim)
-Suicidal Tendancies
-And of course Slayer
If I'm writing a paper:
-Pink Floyd
-Folk/rock like Bob Dylan, Johnny Cash, Arlo or Woody Guthrie
-Neil Young
-Zeppelin
-Hypnotic Brass Ensemble (jazz helps to write)
If I'm reading or studying:
-No music. I can't focus or read when music is pumping into my brain.
I like to keep it chill when I study-pretty lights, thievery corporation, ratatat, rjd2, explosions in the sky, boards of canada, sigur ros, and, to mix it up, some good latin/flamenco guitar type music like rodrigo y gabriella, paco de lucia, etc.