I never watched any machinima really, but I think
Red vs. Blue (using the
Halo game engine) is a popular one.
I've posted my results with
Tyrian and
Rise of the Triad in MobyGames forums. On a side note
Tyrian sometimes literally freezes DOSBox on exit - previously I thought it was only a v1.0/1.1 thingy, but it turns out it also happens in v2.1 and
Tyrian 2000 shareware as well.
I didn't know
Duke Nukem had user demo playback.
Frenkel's site and
Duke Nukem Central don't seem to have any info on that.
BTW, have you looked into
Duke Nukem II?
[Edit] The option to record user demos in
Duke NUkem is mentioned
here, apparently someone was able to actually record a new demo, but the details are rather sketchy.