I don't know that I can explain this fully here, but let me sum up: Joss Whedon (Buffy) and friends got together during the writers strike and this is what they came up with. It's an internet-distributed show in three acts of around 15 minutes each. It's a musical. It stars Neil Patrick Harris as Dr. Horrible, a would-be mega villian with a crush on a lovely ingenue. Don't miss Nathan Fillion as Captain Hammer, a superhero, and Dr. Horrible's nemesis.

I'm not doing it any justice, so I'll say "it's lots of fun" and leave it at that. Visit Dr. Horrible directly or download the acts from iTunes.