Adzeth: As I see it, full conversion mods like NEOTOKYO, Counter-Strike, etc. are games, but I would've taken Half Life 2 as an answer too. :p
Smannesman: By their very definition mods are modifications of existing games, not new games.
You can call a human skull an apple, but you're still not going to be able to purchase one in a grocery store by that name.
From my opinion, the only one difference between a full conversion and a game is the license. If the dev team buy the Source license, it will become officially a game. But when you're playing a mod like NeoTokyo, trust me, you're not playing Half Life 2 or "ordinary" HL2 mod. So, for me, it's an other game.
Good luck with the new guess, because my draw skill isn't a gift :-D