Yeah. Done so quite much actually. More when I was younger than now though. I did so for 4DSD (stunt cars), Wolf3D, Duke3D, Doom, Quake3 (although this one I never got to work properly), CoD1, CoD2, Knytt Stories (obviously) and probably another hand full that I'm forgetting about, not to mention for other systems like the C64.
Have you ever had the following: You were working on an assignment, somewhere during the development, it appeared that it was no longer necessary or otherwise got cancelled, and you decided to just continue the work for the fun of it?