Dr. Horrible’s Sing-Along Blog 9 p.m., Channel 56
Have you seen this 2008 online classic, from Joss Whedon? Tonight, it’s making its broadcast debut. The musical is about a wannabe supervillain played by Neil Patrick Harris (pictured), his nemesis, Captain Hammer (Nathan Fillion, a.k.a. “Castle”), and their shared love interest, Penny (Felicia Day). This is must-see, and Whedon has announced that production on a sequel will begin next spring.