Source: PlanetHalfLifeBBC News has put online a interesting article about Valve banning about 20.000 Steam accounts just recently, for using illegal game versions and other illegal things. Here some words:
Game maker Valve shut down the online accounts of the players because it had evidence that their copy of the game had been obtained illegally. Copies of Half-Life 2 had been circulating on file-sharing systems soon after it was officially released. Experts said the success of the Half-Life 2 anti-piracy system might tempt other game makers into creating their own version.The full article can be found here.