I recently discovered Mutant Future
, the free retro-rpg along the lines of Gamma World 1st & 2nd edition. I spent a lot of time playing GW in 2nd,3rd, and 4th editions and I always had a good time - even character deaths usually managed to be memorable in some way. MF is a worthy successor. The character types are similar, there are long lists of randomly-rolled mutations, and combat is old-school descending AC. If you enjoyed GW I recommend it.