Forsaken / Project X
Forsaken, often known as the Descent clone, was released open source and is now available as Project X. I recommend Project X over picking up a used copy of Forsaken because Project X correctly supports widescreen and includes high resolution textures. You can download it here:
To disable the hud, press F7. This resets each time you run the game.
Some hud elements (like timers) are not disabled. To completely disable the hud, you'll need to download some replacement textures. Put them in your forsaken/data/textures directory. Here's a link to my Skydrive. Download the Forsaken-nohud.zip file: