During the shots when the fan club is sitting in the audience you'll notice the fourth seat is occupied by a woman with sunglasses and a black wig whose nose looks just like Sarah Michelle Gellar's.. because that's exactly who it is. (Confirmed in the DVD Commentary)
Both Doctor Horrible and The Big Bang Theory include an Only One Name character named Penny that knows/is known of by a dork played by Simon Helberg and pined over by that dork's also dorky friend. The resemblance stops there.
No Export for You: In some places outside America. Good thing the DVD isn't region-locked.
Recycled Script: There was an episode of Angel about a shy scientist who liked to sing and was in love with a red-head. He built a machine that could stop time to solve his relationship problems.
Angel also had a plot in Season 5, in which the death of a character was used to get into an evil organization. Angel pretended to be involved in his friend Fred's death and really murdered another to infiltrate the Circle of The Black Thorn and Dr. Horrible's involvement in Penny's death filled the qualification to enter The Evil League of Evil.
Viral Marketing: The word spread about Dr. Horrible by word of mouth and word-of-internet. So fast and so far that the server was not prepared for the load and went down.