It turns out some people put their down time during the recent writers’ strike to good use: Buffy the Vampire Slayer creator Joss Whedon got around the TV- or film-writing prohibition by penning a musical for the intertubes. Because that’s what you do, right? The result is “Dr. Horrible’s Sing-Along Blog,” a 75-minute superhero spoof starring Neil Patrick Harris (!) as a maniacal supervillain, or at least someone who wishes he was a maniacal supervillain. The miniseries was produced on the cheap, using affordable (and sometimes real) locations, and Whedon fans will be pleased to know it features some old Buffy, Angel, Serenity and Firefly cast and crew. You’ll be able to watch it for free at the Dr. Horrible web site, but the schedule is a little tricky: Act One debuts tomorrow (Tuesday, 7/15), Act Two starts Thursday, July 17th, and Act Three will be posted Saturday, July 19th. The videos will stay up on the site for free viewing through Sunday, July 20th, at which time Whedon promises they will “vanish into the night like a phantom.” Or, maybe they’ll be available on DVD or something.