I'm willing to bet that there are games out there that have been developed, printed and purchased, but have never actually been played except by the people that wrote them.
Games I played once or twice, and would have liked to have played/GM'd more:
Marvel Super Heroes (elegant colour-based charts!)
Fantasy Hero (I loved playing it, but I had no idea what was going on; most humans are just not smart enough to GM it - me included)
Harn (very cool system - again, I'm not smart enough to GM it, but I think I enjoyed playing it; not to be confused with HARP, which is Rolemaster-lite!)
Twilight 2000 (although the past decade in my chosen occupation would make it hard to play now with a straight face)
Millenium's End (ditto as above; plus, unfortunate title - virtually gave the game an expiry date...)
Games I'd like to play before I die, but probably won't:
Rolemaster/HARP (who does play this? It's been around for 30 years, and yet I've never met another human who plays it)
Star Wars (any edition)
Renegade Legion
Battlesystem (1st or 2nd edition)
MERP (Rolemaster rules-lite for the '80s)
Savage Worlds
Alternity
Riddle of Steel
Games purchased but never played or GM'd (and never will):
Lejendary Adventures (sorry, Gary, if you're watching...)
Warhammer 2nd Edition
Rolemaster/HARP
D6 Space/Adventure/Fantasy
Star Frontiers
Traveller
Renegade Legion
Decipher Games' Lord of the Rings
...and many others, boxed away never to see the light of day again...