Garry's Mod. Obtained from Humble Jumbo Bundle recently and I don't think I will ever play it. I can create a gift URL and send it to you. Please note that it requires you to have a Source Engine game in order to play.
The full list of games which meet this requirement:
Estranged: Act I,
No More Room in Hell,
Left 4 Dead 2,
Counter-Strike: Source,
Half-Life 2,
Half-Life 2: Episode Two,
Portal,
Half-Life 2: Episode One,
Left 4 Dead,
Day of Defeat: Source,
Half-Life 2: Deathmatch,
The Orange Box,
Valve Complete Pack,
Counter-Strike Complete,
Portal Bundle,
Half-Life Complete,
No More Room in Hell Steam Release Trailer,
Blade Symphony,
Counter-Strike: Source + Garry's Mod,
Blade Symphony Trailer 2,
Source Multiplayer Pack,
Please don't enter if you don't have any of these games already in your Steam library, it will be useless.