CS 1.6 just pulled a time-travel stunt. Valve accidentally included the prototype of Left 4 Dead called «Terror Strike» in the latest CS 1.6 update, and the community managed to run it. :
CS Online 1.6 - Games online 6games.eu
Download CS 1.6 Roblox Edition
CS 1.6 Online - Play CS 1.6 Online
Counter-Strike 1.6 Is Still Playable and it's Fun! - YouTube