For the past nine to ten years, my favorite online comic has been Sinfest. I can't remember how I came across it, but it was definitely in the early days. In 2002, I was so completely hooked that my husband (boyfriend back then) wrote code so the comic would refresh automatically every morning as my desktop background on my computer.
Here are three of some of my favorite strips. Check out the website and read this article about the artist Tatsuya Ishida and his "satirical webcomic about angels, devils, sex and politics that averages 300,000 hits per day". A collection of the first 600 strips will be released in a collection from Dark Horse June 17, 2009 so check it out at your local comic shop.