There's probably a huge list, but off the top of my head I've never played:
- Any Elder Scrolls RPG
- Any Mario Kart game
- Any Zelda game
- Pretty much any NES / SNES game actually so Metroid, Megaman, Donkey Kong, etc
- Half Life 2, or Portal (not 10 years old, but still - played 30 minutes of Portal 2 though)
- Only ever played one Space Quest game (but did play most of the other Sierra 'quest' games)
- Any Metal Gear Solid game
- Any Silent Hill game
- Any Resident Evil other than the first one
- Ultima I, II, III or V (or IX, but it's not considered good)
- Any Wing Commander game
- Diablo I (played plenty of II though)
- Baldur's Gate
- Planetscape Torment (for shame, I know)
- Any Deus Ex game (though Human Revoution is waiting in my 'to play' pile)
- Only got an hour into the first Halo, never finished it or played any other game in the series
- Any Gran Turismo game