(This post is a part of the RPG Blog Carnival.)
I own a Swedish game called Svenil. It's about pensioners, postmen and other stereotypical misfits going on adventures in Swedish suburbs. I've read it once and never played it.
The problem with games trying to be funny is that they often are a funny read at the expense of being a good game. Svenil has silly classes and skills, but doesn't really explain well what one is supposed to do in the setting. The end result is a game that's not really made to be played.
The opposite is a game that's meant to inspire funny situations, but has a functional system. Being burnt by Svenil, I haven't really checked out those, but... Paranoia? Toon? Just guesses.
One time, I saw the GURPS Discworld book in my local FLGS. I wonder which kind of "funny game" that is.