The only ones I can recall actually PLAYING - even if for just one shot - are:
D&D, basic
AD&D, a.k.a. 1st Edition
AD&D, 2nd Edition
D&D, 3rd Edition/3.5
Alternity
Bureau 13
Gamma World (an early edition)
Marvel Superheroes
Mechwarrior
Paranoia
Runequest
Shadowrun
Space Opera
Star Trek, FASA
Star Wars, d6
Torg
Traveller, classic
Traveller, mega
Traveller, TNE
Traveller, 2300
Vampire
Actually, I'm not so sure about Gamma World. It was "around" our gaming group, discussed and referred to on occasion, but I don't specifically recall that I ever actually PLAYED it. A lot of them were one-shots, which is to say they were tried and abandoned in short order.
Traveller classic we played a decent amount of. The other Traveller flavors were always actually hybrids run by one particular GM. I ran a pretty decent Star Wars campaign for some time but it was pretty high-energy and I couldn't take the pace anymore. Space Opera was actually a hybrid with orgininal Traveller but we played a good campaign of it. Some good games of Paranoia and Marvel.