There are several Comic Strips which exist on the WWW; here are some which I know of:

Dailies

The Dilbert Home Page by Scott Adams
Doonesbury by Gerry Trudeau
The spectacle which is...Dr. Fun, by David Farley
Something of a guilty pleasure, For Better or for Worse by Lynn Johnston
One of my all-time favorites, Mr. Boffo, by Joe Martin
P. S. Mueller's untitled strip
Reruns (no pun intended) of the one of the most famous strips in history, Peanuts by the late Charles M. Schulz
Some editorial cartoons dealing with the U.S. political scene
Some other strips (most of which I personally don't like)

Wednesdays: This Modern World by Tom Tomorrow on the Working for Change website.
Saturdays: Tom the Dancing Bug by Ruben Bolling.