For me its about mood, and the work I have at hand (and the level of concentration it needs)
I listen to many formats: Rock, Alternative, Rap, Slow, and Classical.
Yes, they are all very different.
But I mainly listen to Rock and its different variations.
Bands and singers I like:
Staind
Guns n Roses
Bon Jovi
Sarah McLachlan
Nine Inch Nails
Coldplay
Faith No More
Natalie Merchant
Yellowcard
I'm a sucker for melancholy songs and movies.