Half-Life 2 maker Valve has hit back at players who obtained pirated copies of its popular new game. To be able to play, players must set up an account on Steam, Valve's online gaming community. Steam then verifies whether the player's game is legitimate or pirated. Valve has now shut down close to 20,000 Steam accounts on the grounds that the account holders obtained the game illegally. Valve maintains that this system makes it easy to trace and confirm which accounts use pirated copies, but some banned users claim they are innocent. Clearly there are some wrinkles to be ironed out, but Valve's approach may be a significant new development in copyright protection for video games.