Valve’s first-person shooter game started as a sequel to the original Team Fortress mod for Quake, but the design was adjusted over the game's seven-year development. The final version of Team Fortress 2 appears to pick up where Team Fortress Classic left off, with some great team-based multiplayer action and a cartoon-like visual style.
The Team Fortress 2 beta on Steam includes all six maps, the nine original classes of Team Fortress and a bunch of cool features like revenge points and critical hits. You can read a couple of hands on impressions of Team Fortress 2 here and here. Pre-purchasers of the 5-in-1 Orange Box Steam package will get access to the full game on October 10, in addition to Half-Life 2, Half-Life 2: Episodes One and Two, and Portal.