Halo 3 Epsilon

Halo 3 Epsilon is believed to be a Halo 3 build.

It was released to Microsoft employees as of August 10, 2007, and featured campaign (single-player and cooperative gameplay both), Forge, and multiplayer modes.

Halo 3 Epsilon was confirmed to some degree later on August 10th by Bungie Studios on a Bungie Podcast, and Microsoft employees, Xbox.com representatives, and the Bungie Studios staff have been playing it, and the Bungie employees are using this build to finish the final details of the game.