There's Lightbringer (also called Cydonia) if you haven't played that. If you can get a hold of the DVD version it's a plus (to avoid the disc swapping). I really enjoyed it.
Haven't played it, but I heard a lot of good things about Dark Side of the Moon, again - if you have the DVD version to avoid the tedious disc swapping.
"How could drops of water know themselves to be a river? Yet the river flows on."
- Antoine de Saint-Exupéry