All entries subject to change, since I honestly listen to so much music in so many genres that I know I'll be missing some great stuff because I'm kinda doing this spur-of-the-moment-like. I'll only include genres for groups I feel the majority of you don't listen to/care about/know about....although I do include commentary for all!

#: 311 (best number group I have anything by...by far).
A: Amorphis (Finnish folk/death/progressive metal/rock....depending on time period...they evolved greatly from their early pure death days)
B: Bob Dylan ('nuff said)
C: Cathedral (British stoner metal, influenced heavily by Sabbath, but with a heavier, sludgier sound)
D: Dream Theater (awesome progressive rock/metal)
E: Entombed (Swedish death/hardcore punkish stuff)
F: Faith No More (just when I was ready to give up on this letter for the night, a song by them I put on one of my "I'm bored" mix tapes played...thank you KARMA!)
G: Grateful Dead (awesome "catch a buzz" music there)
H: Hypocrisy (amazing Swedish death metal group that has succeeded in repeated altering their style without ever selling out....ie: becoming mainstream).
I: Iron Maiden (the first metal group I ever listened to and the greatest).
J: Johnny Cash (amazing library of material here)
K: KISS (at least their really old stuff from the 70s....I try to forget most of the rest exists).
L: Led Zeppelin (their versatility as far as musical styles was amazing)
M: Monster Magnet (stoner rock/metal)
N: Neil Young (lot of good stuff).
O: Obituary (the kings of American death metal....their crushingly heavy rhythm guitar sound is just amazing)
P: Paradise Lost (versatile group that's done slow-tempo death before moving on to more mainstream metal styles)
Q: Queensryche (has some great stuff spanning from Warning to Empire
R: Red Hot Chili Peppers (one of the best groups of recent times)
S: Suicidal Tendencies (punky, funky thrash)
T: Tom Petty (always been a fan of his music)
U: Later....
V: Velvet Underground (easily the best "V" group I can think of)
W: the Who (simply phenomenal stuff)
X:
Y:
Z:

I'll fill in the holes and finish off the alphabet tomorrow or Wednesday....my brain's slowing down and I don't want to put blah groups in just 'cuz they're all I can think of.

EDIT 1: Added the Who as my "W" band!
EDIT 2: Changed my "N" pick!!