The Webcomic Crossover and Cameo Archive is one of those internet/webcomic curiosities that you thought might never exist unless you made it yourself. Meanwhile less cautious types ran with the idea, and here we can see our medium's equivalent of Ricky Ricardo running a white lightning racket on Mayberry R.F.D.
Dinosaur Comics, a personal favorite for oddball insertions, is among the most popular with colleagues as well, as is Sinfest. Lots of defunct comics are listed, enhancing historical interest and confusion.
Webcomic artist cameos are also catalogued, with a favorite role apparently being "guy who opens the door on Halloween."