My D&D group played an RPG called Gamma World instead of D&D tonight and it was really, really fun! It's a post apocalyptic theme with really fun, random kind of mechanics. It's related to the fourth edition of D&D using practically the same system (it's owned by the same guys), and works basically the same way, but has a much more spontaneous feel to it. Each encounter tends to be different. At the beginning of a fight, each character gets an Alpha and an Omega card, which gives them a unique element for that fight. The Alphas are mutations, random mutations because the world's all messed up and stuff, and the Omegas are tech, which can be a grenade or a nanobot patch or a Gravity Hammer, they all tend to be pretty fun to use. Some are just plain silly, like the porcupine quills that I sprouted with in one fight, or the long nose that counts as a stabbing weapon that one of the others got later.
And instead of picking a race and a class, you roll for two Origins, which determine certain stats and your abilities. I rolled a Mind Breaker/Plant, and there was a Felinoid/Yeti. Generally fun, random stuff.
Instead of item pickups at the end of an encounter you get Ancient Junk, basically stuff left over from the normal world. There were three horses in our group, plus a wagon and a pickup truck. I had duct tape, so we were pretty much set. XD
The pickup didn't have doors on it, so we used a table saw we found with one of our generators and ripped the doors off of a building we found and slapped those on the truck. We decided we were going to turn the truck into a massive RV with everything made out of doors. Lots of silly fun, despite how we were heading towards a TPK in our last fight.