So many....
Shadowrun I miss the most. First and Second edition. Third was okay... I got the most compliments running this game.
2300AD by GDW. I'm surprised nobody mentioned this one. Probably because it was a near direct rip of Traveller, but who's keeping track?
Torg was always a cool idea. Something about a Nipponese Assassin facing off in a 1930's pulp style era was just so.. fun.
The original Talislanta. No Freaking Elves, especially drow. Rant aside, I thought it was a cool system and if you look closely, it smells of 3.5....
Star Wars d6. Great system, great GM. Bad players..... Too many Jedi.
Vampire... my 8th Generation Tremere is sorely missed in Colorado...
Werewolf... Shadowlords just rock....
Mage... Akashic Brotherhood all the way...
Star Frontiers... When I saw Dralasite in d20 Future, my jaw dropped. Now I await d20 Starships.... (hint hint)
I never played Top Secret, but always wanted to. So many ads and articles in my old Dragons got me interested, but I never had the chance to buy it and check it out. The same with Villians and Vigilantes.
As much as the Palladium system bugs me, PFantasy, Heroes Unlimited and Rifts still have special places at my gaming table since they were what brought me to friends I've had for 14 years now. It's too bad we all live in different states...
There are more systems that I once owned and never got around to playing, but for brevity, I'll let it slide. For now...
Edit: Oh, and GURPS. Something about a 500 point cinematic martial arts campaign came to mind... And Toon by Steve Jackson as well. There was some fun times....