Boomtown have got an article which takes a look at WebComics, called Webcomics - The fun is out there. The article takes a look at webcomics in general but also focuses on some of the most popular such as Penny-Arcade!, Player vs. Player and User Friendly:
Since the nature of the internet allows practically everybody to spread their art and crazy thoughts, quality is of course varying. The best of these comics, though, range up there among the paper-based type you normally have to cough up money for or have to go to the library to read. If you don't mind reading the comics on a screen - or printing them out for that matter - there is a goldmine of high quality strips on the web free of charge.
Since most of these online artists work by their own schedule the number of updates on the different sites can differ. The standard for most, however, is about four or five times a week, making it a good idea to return often. Besides the strips most of the cartoonists also give a personal 'rant' from time to time on their comic site. This can be a view of everything ranging from the deeper meaning of the shape of hamburgers to the true nature of the universe. In some cases these rants are almost as funny as the strip itself.