Series:
Girl Genius by Phil and Kaja Foglio - Excellent art, excellent stories, great humour.
Buck Godot: Zap gun for hire by Phil and Kaja Foglio - A earlier work by the creators of Girl Genius. The complete series is online.
Schlock Mercenary by Howard Tayler - Madcap adventures of a band of mercenaries. Pretty funny, the initial strips are a bit rough in terms of the art but all in all this is good stuff.
Strips:
XKCD by Randall Munroe - As the site says a webcomic about romance, sarcasm, math and language. Simple looking art but a great comic nevertheless.
Penny arcade by Jerry Holkins and Mike Krahulik - You need to like computer games to get the context in many of these strips. One of pre-eminent gaming related web strips.
