NPR had a piece today about the videos on Dr. Horrible's Sing-Along Blog that were released earlier this week. The first two episodes are posted now and the third one will be posted on Saturday.
According to this SFGate.com article you can watch Dr. Horrible free until Sunday night, after that you have to buy it on iTunes or wait for the DVD.
Here's a link to a YouTube video on Wired.com from a fan who wants to be Dr. Horrible's sidekick.