I've now seen what was obviously part of the beginning of the Dr. Horrible concept. We've been watching season one of "How I Met Your Mother" and on the episode called "Game Night" the tale is told of how Barney (Neil Patrick Harris) changed into the rather despicable guy he is now. Some of the music was nearly the same as the Dr. Horrible theme, and the scene where "Barney" puts on a suit is very like when "Billy" puts on the red lab coat and black gloves.
Of course, "The Trio" in season 6 of Buffy is the other part of proto-Horrible. I can't help thinking, though, that the whole thing was written for Neil Patrick Harris after seeing him as "Barney". There's even an episode where he did an evil "Muuuuaaaahhhh haaaaaaaaaaaa haaaaaaaaa!" laugh early in season 1.