Recently I've discovered a game called
Subevel Zero, which is a 6DoF
Descent-like corridor flyer/shooter with randomly generated levels. I checked if a playable demo was available, and found out that apparently this game was originally created for
Ludum Dare, and a pre-release free/trial version is available from
here.
The free version is quite different from the final commercial game, and it's somewhat closer to the original
Descent as well. It allows for infinite play with randomly generated levels, and there are also random powerups that enhance the player's stats (e.g. increased rate of fire or maximum ammo capacity for a certain weapon). Here's a couple of gameplay videos:
Ludum dare entry gameplay
Commercial version gameplay