10. An Extra Musical - Dr. Horrible's Sing Along Blog
Dr. Horrible's Sing Along Blog follows the trend of musicals having people sing where they shouldn't with a determined focus. So much focus, in fact, that even its DVD commentary track is entirely comprised of songs.
Named "Commentary! The Musical", this feature contains an extra whopping fourteen songs, which range from actually discussing the film itself, to just being the most meta creations music has possibly ever seen.
There's something so delightful about the fact that this was slid into DVD copies without much fuss. The idea of someone, having watched the film, idly slipping on the commentary tract only to find a full extra musical is both surreal and delightful. When you think about the amount of time it must have taken, it's absolutely insane that everyone opted to go the extra mile to create something that only those who browsed extra features would enjoy.
Nowadays, you can buy a copy of Commentary! The Musical itself. It really says something about the level of work put into the film that even its commentary is impressive as an entity all on its own.