I started wondering how many table-top RPG systems I'd tried/played in. So I made a list. Edition numbers mentioned where I could remember them, omitted where I couldn't. Non-commercial homebrew systems omitted. Recent as of 7th of September, 2008.

Played = Played the game
Played* = Played the game (GMless system)
GM = Game mastered the game

Ars Magica (4th edition) GM, Played
Bacchanal Played*
Big Eyes, Small Mouth Played
Call of Cthulhu D20 Played
Dungeons & Dragons (3rd edition)
Dungeons & Dragons (3.5th edition)
* Greyhawk campaign setting Played
* Dark Sun campaign setting GM, Played
* Forgotten Realms campaign setting GM, Played
* Ptolus campaign setting Played
Dungeons & Dragons (4th edition) GM
Fastlane Played
Exalted Played
GURPS (3rd edition) GM, Played
Little Fears Played
MechWarrior Played
Middle Earth Role Play Played
My Life with Master GM, Played
Paranoia Played
Rolemaster Played
RuneQuest Played
Star Wars (2nd edition) GM, Played
Traveller Played
Vampire: The Masquerade Played
Vampire: The Requiem GM
Warhammer 40,000 Roleplay: Dark Heresy Played
Zombeja! Ovella! Played*