Penny Arcade
http://www.penny-arcade.com/comic/

Gamer humor.

xkcd
http://www.xkcd.com

Funnies for the hardcore geek in you.

Dilbert
http://www.dilbert.com/

Everyone's favorite cubicle worker.

Achewood
http://www.achewood.com

Absurdist, almost surreal funnies from the fringe.

White Ninja
http://www.whiteninjacomics.com/comics.shtml

Poorly drawn, sometimes in poor taste, often funny.

LOLcatz
http://lolcatz.wordpress.com/

Can I haz a funny cat?

Boing Boing
http://www.boingboing.net

Not a comic strip, but we couldn't resist putting it in here.