Starring Neil Patrick Harris, Nathan Fillion and Felicia Day, the first part of the three-installment comedic short debuted late Monday night, while part two is to premiere Thursday and the saga is to conclude with part three Saturday, Variety.com said.
Whedon and Fillion previously collaborated on the TV shows "Firefly" and "Buffy the Vampire Slayer," as well as on the feature film "Serenity." Harris is best known for his roles on the TV series "How I Met Your Mother" and "Doogie Howser, M.D."
The first part of the "Sing-Along Blog" was released as a $1.99 download on Apple's iTunes or free Web streaming on Drhorrible.com. It ranked No. 1 on iTunes TV downloads for most of the day Tuesday, the entertainment industry trade paper said.
Shortly after the first part aired, the "Dr. Horrible" Web site crashed because it was getting 200,000 hits per hour. However, Whedon's brother Jed, who also co-wrote the made-for-the Web musical, then secured a larger server to handle the traffic and got the site running again, Variety.com said.
The whole musical will be available for free streaming through midnight Sunday and through July 29 on iTunes.