Here's a YouTube site devoted to converting the original Marathon Vidmaster films to video.
Again some of these are videos of original Marathon gameplay. Vidmaster films by their nature don't show normal gameplay but they are fun to watch nevertheless. If you want to see Jason Jones, Miha or Degu in action on the original Marathon levels this is probably your best shot (unless you can play the original Marathon films from the Vidmaster page).
If you only have time to watch one video then it is this one.
Miha takes on the Deprivation Chamber on Never Burn Money. Unfortunately the look and feel of the Deprivation Chamber cannot be implemented in Aleph One but one day perhaps the coding Gods may prevail. :)