A long time ago there was the Bench, a place where fans could make webcomics using stock characters from Penny Arcade artist Gabe (and many mixed in their own creations as well).

More recently, the king of the webcomic madlib is Strip Creator, an automated webcomic generator featuring art from several webcomics, including Jerkcity and Exploding Dog.

Strip Creator is now over two years old and continues to add features to its site. A site worth visiting, even if you've been there before.