Steam

Steam has been around since 2003, and in February 2009 it passed the 20 million mark in user accounts, making it one of the leading content delivery platforms in the world. Boasting 837 games from a who's-who index of publishers- including 2K Games, Activision, EA, and Ubisoft - Steam is where Windows gamers turn to buy everything from casual time-killers to enthusiast-caliber thrills.

Steam currently has over 25 million active users, largely thanks to the availability of the hugely popular games Counter Strike: Source, Portal, Half Life and a number of smaller, independently developed titles.