Well I came across this via Aaron William's blog (points to the right) and I was a little ashamed of myself that I wasn't already aware of it. Dr. Horrible is a three part series (I'm not sure if more are planned after this) from Joss Whedon starring the excellent Neil Patrick Harris as Dr. Horrible and Nathan Fillion as his arch rival Captain Hammer.

Here's the trailer

Ok, so it'll be shown in three parts I believe on the 15th, 17th and 19th of July and then the entire thing will be available to watch online on the 20th (which is the final date for the free airings) - there will be a DVD version (with extras) later. The creator, the cast, the subject and the fact that it's free makes this a must watch.

Sites:
Doctor Horrible's Sing-Along Blog
The Official Site