In response to the writer's strike, Joss Whedon is trying out a new method of distributing work without Hollywood. So, for this week only, Dr. Horrible's Sing A-Long Blog will be online for your viewing pleasure. The first act came out on the 15th, the second act came out today and the third act will be out on Saturday. You can also download them from iTunes, I believe. The whole thing will be taken down after midnight on Sunday, possibly to be made available to download for a small fee or to be put on DVD at a later date.
Plot description, from the myspace page:
The story of a low-rent super-villain, the hero who keeps beating him up, and the cute girl from the laundromat he’s too shy to talk to. Featuring Neil Patrick Harris as Dr. Horrible, Nathan Fillion as Captain Hammer, Felicia Day as Penny and a cast of dozens.
It's a musical, too. And it's very funny. Go forth and watch!