A lot of Metal and the stem-genres from it (Spineshank, Metallica, Marilyn Manson, White Zombie, Element 80, Lamb of God, Judas Priest, Iron Maiden, Paradise Lost, Cathedral, Suicial Tendencies, Disturbed, Sanctuary, Slayer etc...) to classic rock (Aerosmith, Pink Floyd, CCR, ZZ Top, Charlie Daniels, etc...) to more trippy/techo/hybrid stuff (Garbage, NIN, Mono, Chemical Brothers or remixes of other genres etc...) to blues (B.B. King, Muddy Waters, Johnny Lee Hooker, Stevie Ray Vaughn, Jimmy Vaughn) to classical (Tchaikovsky mostly) to rap (DMX, Ice Cube, Top Authority, Eminem, Lil John etc...) to poppy stuff (Allanis Morrisette, Avril Lavigne, Bare Naked Ladies etc...) to classics (Johnny Cash, The Coasters, Cab Calloway, Fats Domino, Otis Redding etc…).
Honestly, it really depends on my mood, I can go from wanting aggressive and fast moving music (metal, DMX etc...) when I’m working on my guns or doing computer work to chilling out and having some scotch listening to classical or blues to grilling out and listening to Clapton and the Beatles while tossing back a few Rolling Rocks or Corona’s....
My MP3 player that I have in the car and take with me most places I go has 1856 songs on it (almost 8gigabytes-worth) and has something from just about every genre and cross-genre except bubblegum boy bands (N-SYNC) and overly religious themed music. I just put it on random and go with the flow most of the time. The playlist is probably 60% metal or other heavier-type music, if that gives you an idea as to my most common preference.
Right now it's playing "Bullet With a Name on It" from Nonpoint and oddly enough, it just finshed playing "Vote With a Bullet" from Corrosion of Conformity.