What are you waiting for? Apogee/Interceptor's 2013 remake of Rise of the Triad is currently free on GOG. Go and get it before the 30,000 copies being given away are gone.
If you want to stick around to learn what it is: it's a recreation of a 1995 FPS in which five distinct characters can be chosen to take down a cult known as the 'Triad'. To be honest, I'm not a massive fan, but our reviewer liked it well enough. The multiplayer's pretty good, if mindless old-school shooting is your jam.
Nonetheless, it's free, and you can get it right now—assuming that reading all of this hasn't scuppered your chances. If it has, the game is 80% off this week—alongside Unreal Tournament 2004 at 70% off, and AvP Classic 2000 at 50% off.