Half-Life 2

Troubled by many delays and even hacker break-ins to steal source code the game finally released on August 26, 2004.  It was almost an immediate success.  The game was developed on the new game engine developed by Valve called the Source Engine.  Later releases built on this game engine include Counter Strike Source, and Day of Defeat Source. 

Half-Life 2: Episode 1

The first Episode of the Half-Life 2 trilogy was released on June 1, 2006 as a standalone game, and is now included in the Orange Box.  The game received IGN's Best PC FPS of 2006, describing it as "great bang for the buck using Valve's new episodic plan."  Episode 1 sports a more team work driven experience as you battle along side your new companion, Alyx Vance. 

Half-Life 2: Episode 2

Episode 2 aims for a more rural environment.  Beginning in the train wreck subsiding outside City 17, Gordon Freeman and Alyx Vance find themselves sanctioned in forest environment, full of Xen wild life previously kept at bay by Combine counter measures.  It incorporates the use of a Hot Rod to drive through a large part of the game.  Half-Life 2: Episode 2 was released with the Orange Box on October 10th (USA). 

Portal

Portal is a single-player, puzzle game developed by Valve and released in the Orange Box.  Single-player will take you through a series of simple and complex puzzles.  You must solve each puzzle by opening up "Portals" with the Aperture Science Handheld Portal Device in order to teleport yourself and simple objects such as cubes, or even bullets.  Complete the game to get the cake. 

Team Fortress 2