Best roguelike ever, in my oppinion, is ADOM. Especially using the original ASCII graphics. It has more depth and content than any other game Ive encountered. And the original version is still completely free (paid version adds pixel graphics and some advanced options)
You dynamically board spaceships, sneak around them, complete objectives, and extract before you're captured. There's a really cool time stopping mechanic so you can do things like "Shoot at this guy but throw this object at that guy" without needing real-life impossible reflexes.
It's got a ton of really cool ideas and I had to force myself to put it down and take care of Real Life Stuff. :p