Valve's innovative "VAC" (Valve Anti-Cheat) has gone through two iterations -- the marginally successful VAC1 and the industry-leading VAC2 -- since its implementation into the classic online shooter, Counter-Strike, way back in 2002.
The service has done so well that, if one was to name any of the most popular PC shooters right now (Left 4 Dead, Team Fortress 2, Counter-Strike, etc.), there is a good chance that it is being "watched over" by Valve's VAC client.
Read more - Are Console Developers Doing Enough to Punish Cheaters?

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