It’s by Joss Whedon (of Buffy and Firefly fame), it’s a musical, with super-villains (kind of), super-(smarmy)-heroes, and it’s free. Dr. Horrible’s Sing-Along Blog is online – the first part already viewable, with the second and third acts arriving on the 17th and 19th of July. After that, you’ll have to pay to see them through iTunes, or wait for the DVD.