I couldn't pick only one for some letters.

#-7 Seconds
A-Alkaline Trio
B-The Bronx/Blood Brothers
C-Cursive/Johnny Cash
D-Descendents/Dillinger Four/DRAGONFORCE
E-Easy-E
F-Face to Face
G-Green Day
H-Hot Water Music
I-The Impossibles
J-Jughead's Revenge
K-Kid Dynamite
L-Lagwagon/The Lawrence Arms
M-Minor Threat / Me First and the Gimme Gimmes
N-NOFX(my favorite band of all time)
O-The Offspring
P-Pennywise/Pulley
Q-Queen
R-Rancid/Refused/Reggie and the Full Effect
S-System of a Down
T-Tenacious D
U-Useless I.D.(Really good pop punk from Israel)
V-The Vandals
W-A Wilhelm Scream/Weezer's first two albums
X-??
Y-Youth Brigade
Z-Nothing