In no particular order:

• User Friendly (is getting quite stale; I’ve been reading it since 2001 or so)
• Wapsi Square — girls, golems, Atlantic artifacts, etc
• Questionable Content — college kids with issues :-)
• Girls With Slingshots — post-college kids with issues :-)
• XKCD — you know this one, of course
• MegaTokyo — ditto

Also:

• Inverloch — fantasy stuff
  
• The Order of the Stick — stick figure D&D
• Unicorn Jelly — (finished) surreal and pixelated
• Hackles — (finished) kind of like User Friendly with animals
• Haiku Circus
• The Joy of Tech

On a side note, both QC and WS have shown tremendous change in art quality over time. (Compare the first comics with recent ones to see what I mean.)

If you know any good webcomics, feel free to recommend them in the comments. ^_^