
Descent 3 has been made open source
Another classic has been given the open source treatment, with Descent 3 from Outrage Entertainment now available under the MIT license.
D1 and D2 also have source ports, for instance D2X-XL: Descent 1 and 2 revived and enhanced!
Descent was my very first 3D game; it came preinstalled on my first computer in 1995.
I never finished it back then because it was too scary.

Also, it was entirely in English, so I had no idea what was going on, and figured out the controls through pure trial and error. Good times.