XSp.

Dunno if I should call this Abandonware, since Doom, even being more than 15 years old, is still on the mind of lots of people:

One of the first FPSs to try “realistic” 3D graphics, and also one of the games that made FPS a standard among genres of games.
I played it… too much. I didn’t get to finish it in nightmare mode, but I went far… and finished it tons of times in Hurt me Plenty. 😛
Still goes back to when mouse was not used for FPSs, and it took me a long time ‘till I learned how to play FPSs with a mouse mainly due to the fact that I played too many FPSs without it (I skipped Quake, and only started playing FPSs again with Quake 3 Arena).
Doom was awesome. I remember lots of scare-jumps when I played it at night, with all lights out, and this huge amplifier under my desk. xD