I really dig the show
Ugly Americans. It's is set in an alternative New York city peopled by monsters, wizards, mutants and demons. It's like the typical FRPG populace in an urban setting. Here's a taste.
As I see it, the PC's would all live in the same big city, and all share the same menial job, like taxi driver, security guards or working at a
Rat On A Stick franchise. As for the rules set, the obvious choice would be
Toon. There's already a fan made
Futurama game using
Toon floating around out there, so having an
Ugly Americans one too is only natural. But another good choice would be
Goblinoid Games' Labyrinth Lord and
Mutant Future.
LL has all the monsters, wizards and zombies you need for the game, while
MF has the stats for the robots, mutants, etc. A few minor tweaks and your ready for downtown weirdness. With the right players I think it would make for a great, off-beat campaign.