Dr. Horrible's Sing-Along Blog is a web original movie created by Joss Whedon, Jed Whedon, Zack Whedon, and Maurissa Tancharoen that first debuted in the summer of 2008. It tells the story of nice-guy supervillain Dr. Horrible (played by Neil Patrick Harris), and his attempts to win over the girl of his dreams, Penny (Felicia Day). Along the way, he tries to get into the Evil League of Evil and defeat the jerk of a superhero Captain Hammer (Nathan Fillion). Oh, yeah, and it's a musical.
Dr. Horrible's official website is here.