Way too many. Things I own but haven't played at all:
Arcana Unearthed/Evolved
Talislanta 4th ed (though I bought that about a month ago, so it doesn't really count)
Buffy
Hackmaster
Adventure!
Drakar och Demoner 6th ed (Swedish RPG - I've played earlier versions of it, but the 6th ed is about as different from those as D&D 3e is different from AD&D)
Everway (got it as a gift about a month or two ago from a friend who was clearing out his RPG stuff, haven't had the time to absorb it yet)
Marvel Super Heroes (old TSR version, not the one with cards)
Basic/Rules Cyclopedia D&D
Star Wars d20
Things I've only played like a single session of:
Exalted
Earthdawn
Masterbook/Bloodshadows
Eon (Swedish fantasy RPG)
Macho Women With Guns (non-d20)
If you include specific D&D settings, add Birthright, Mystara, Jakandor, and Al-Qadim to the first list, and Spelljammer, Red Steel/Savage Coast, and Planescape to the second.