Halo 3 Epsilon
From Halopedia, the Halo wiki
Halo 3 Epsilon[1] is believed to be a Halo 3 build.
It was released to Microsoft employees as of August 10, 2007, and featured Forge and multiplayer modes[2]. While it has not been officially confirmed in a news release from Microsoft or Bungie Studios, several people have been seen playing it, including Bungie content manager Frankie O'Connor, Xbox.com representative "Major Nelson", and a handful of Microsoft employees.