Halodoom is a project about the past and the present colliding. It is a new story set in the Halo universe told using the Doom engine.

Halodoom uses the Zandronum multiplayer port for Doom by id Software to bring a new yet familiar experience. Halodoom comes complete with campaign, multiplayer and firefight. Halodoom is over seven years in the making, and fuses all of the things that made both Doom and Halo great.

The project page can be found here: http://forestbear.net/projects/halodoom

It is currently in Beta.