As a terrorist vs counter terrorist game, this game has consistantly been a favorite of the game nights, whether playing the regular version of the game, or the Gun Game mod.  One of the core games along side Half-Life 2: Deathmatch, if you don't have this game you can pick it up at the Steam Store.  We recommend getting the "Source Multiplayer Pack" which provides both CS:S and HL2:DM.