I'm not aware of many (one of the best Slayers doujinshis shut down because the writer dumped it). For the sake of posting something, I'll put up a link to a very popular one that features characters from a lot of cartoons on Cartoon Network and other stations. This guy's artwork is just AWSOME.
http://bleedman.snafu-comics.com
It's mainly about the PowerPuff Girls, but there are many other characters in there. I don't really like the PPG myself, yet this makes them interesting.
Check out his artwork on his Deviantart page (the link can be easily found on his homepage) to see his version of other characters that haven't appeared in the Doujinshi yet.