These should be pretty close in order of popularity, at least for the date of this post.
xkcd
xkcd.com/
Stick-figure strip featuring humour about technology, science, mathematics and relationships, by Randall Munroe.
Penny Arcade
www.penny-arcade.com/
Equal parts comics and commentary, Penny Arcade features
Dilbert Zone
www.dilbert.com/
Dilbert and gang offer [...]