I started playing Gamma World in 1995, starting with the 1st edition, and moving to 3rd edition about a year later. (I collect TSR games, and always try them out.)
I started on the Alpha to Omega campaign (named so because every adventure in the series had a roman letter as part of it's name, Alpha Factor, Beta Principle and so on) that was published in support for the 3rd edition in 1996 and finished the final published adventure, Epsilon Cyborgs last year. This summer we started on the final adventure in the series, which I'm writing myself and which features one scenario for every remaining letter of the roman (or greek) alphabet. It should be finished by next summer.
I've also run a few adventures in Dark Sun, including the one included in the first box and Black Flames. (As an aside, most AD&D-worlds are post apocalyptic but set some time after the catastrophe and DS is the one that feels truly postapocalyptic.)
I've played in a Twilight 2000 campaign but it broke down due to uninterested players and a DM that couldn't really manage the group we had then.
Another game I played in was the Swedish Gamma World rip-off, Mutant which ended because the DM wasn't that interested in GM'ing. Mutant was essentially GW with the Basic role-playing system and set in Mutant Scandinavia (Finland, Sweden, Norway, Denmark).
And finally I've been in a Wastelands game, It's another Swedish game of a Sweden (and world?) occupied by an alien race. The look and feel was very inspired by the Mad Max movies and we played in what remained of the town were we players lived. It used a system that I've read is similar to the one Fantasy Games Unlimited used.