My group is ADD when it comes to game systems. I can say that, because I've got ADD. Anyway, I'm currently playing in:
D&D 3e, core setting
D&D 3.5e, Dragonlance CS
With games planned for:
Palladium Fantasy
D&D 3.5e, homebrew Dark Sun funness
And planning on running a game in:
D20 Modern, futuristic homebrew franken-setting
And have played in:
Vampire: The Masquerade
Vampire: The Dark Ages
Werewolf: The Apocolypse
Mage: The Ascension
Rifts
AD&D 2nd Ed
Either AD&D 1st ed, or OD&D, can't remember which
Star Wars, West End Games style
D&D 3.5e, Dystopia Campaign Setting (beta runs)
And many more...