I am currently using Savage Worlds, which is basically a Miniatures rules set with a RPG bolted on. There are several fan written takes on Post Apocalyptic goodness. Not Savage Showdown is just the miniatures rules and is available from here;
http://www.peginc.com/shop/savage-worlds-showdown/Both rules sets pretty much seamlessly integrate with each other, the biggest difference is in the vehicles section. I use Savage Showdown mostly pulling out Savage Worlds for specific characters.
40k:RogueTrader (through 2nd ed), Necromunda, GorkaMorka and the Ashwastes Net book are another route one could take. I have done so in the past.
Another group I know of uses em4's Combat Zone with great glee. I haven't seen this rules set, but reports it works well for them.
http://www.em4miniatures.com/acatalog/Combat_Zone_products.htmlIt really depends on what scale of conflict you are going for, in my crowd we are focused one the War Band/gang level so most of our games are quasi-RPGs, or what I like to refer to as Roll-Play. The other bit is what elements do you need to include to be happy? Meaning is all infantry action, other do you want Vehicles, mutants, zombies and other assorted weirdness? And lastly how much effort do you want to put into your setting and rules? In short if you have a favorite rules set then it generally doesn't take much to bolt on the Post Apocalypse to them.