I'm surprised that no one mentioned Excel Saga. The anime series for that is hilarious. I'm not sure about the manga. I heard that both are pretty different.
Abenobashi Mahou Shoutengai is funny too, and so is Jungle wa Itsumo Hale Nochi Guu. All of these titles have a US release now so they should be pretty easy to find.
Oh, and one of the funniest dubs I've seen would definitely have to be...Samurai Pizza Cats!
