It is a web browser that has been optimized to display detailed information about the newest happenings in the world of videogames. You have direct acces to online servies such as FilePlanet and WebGames on the program's interface.
GameSpy allows you to locate new game demos, find servers for multiplayer games, chat with other users of the program, etc.
Currently, the number of games that GameSpy supports is more than 550, including popular titles such as Half Life 2, CounterStrike, and Battlefield 1942.
It's a shame, however, that in order to enjoy all of GameSpy's functionality you have to purchase a paid subscription.