There have been three versions of Halo: Combat Evolved made. There is the original Xbox version, the PC/Mac version, and the Xbox Classics version.
Xbox[edit]
Halo: Combat Evolved was originally released on the Xbox as a launch title. This featured the campaign mode, and the multiplayer that made this game famous. When released, Xbox Live was not yet up, so the game only supports system link with up to 16 players, or splitscreen with 4 players.
PC/Mac[edit]
While Halo: Combat Evolved was an Xbox launch title, it was originally developed for the PC/Mac. In 2003, Gearbox was asked to port the game to the PC/Mac. The game still contains everything the Xbox version had (Besides co-op), but has a few additions to multiplayer. New weapons like the Fuel Rod cannon and the Flamethrower were added, as well as being able to fly a Banshee in multiplayer. The biggest addition was the ability to play online, which can allow up to 16 people per server.
Xbox Original[edit]
The Xbox Original version is simply a port of the Xbox version. There are no new additions to it, besides it being Xbox Live Aware. It is for sale on the Xbox Live Marketplace for 1200 MS points.