If, like me until recently, you haven't seen or listened to Dr. Horrible's Sing-Along Blog, I will make this quite simple. GO WATCH IT! At the very least, listen to it. It is on youtube for crissakes. However, I imagine such people will be few and far between, but if you're still waffling about it, let me try to convince you of the brilliance of this form of aetheric entertainment.
Doctor Horrible's Sing-Along Blog is/was an internet series from back during the Writer's Strike. Remember? There wasn't anything new on TV? Well, our good friend Joss Whedon and his brothers got together with some friends (namely Nathan Fillion, Felicia Day and Neil Patrick Harris) and made a short web series. The "Blog" is divided into three acts, each about a quarter of an hour, so it isn't a huge timesink to watch. The series follows the escapades of Dr. Horrible (played by Neil Patrick Harris) as he attempts to join the Evil League of Evil. Horrible is consistently foiled by his nemesis Captain Hammer (played by Nathan Fillion, Mal Reynolds of Firefly). We are also introduced to his obsession with a girl who does laundry at the same laundromat at he does, Penny (played by The Guild's Felicia Day). Have I mentioned it's a musical? Between bouts of plotting, plot and scheming (all different things I assure you) we have sections of each episode (roughly three per act) that are sung and my are they brilliant!
It's just a really good series, and I'd hate to spoil it for people, so if you haven't already seen it, GO DO SO! THE INTERNET COMMANDS YOU!