What is it? Dr. Horrible's Sing Along Blog.
What Joss has to say about its creation in his own words from whedonesque.com:
Once upon a time, all the writers in the forest got very mad with the Forest Kings and declared a work-stoppage. The forest creatures were all sad; the mushrooms did not dance, the elderberries gave no juice for the festival wines, and the Teamsters were kinda pissed. (They were very polite about it, though.) During this work-stoppage, many writers tried to form partnerships for outside funding to create new work that circumvented the Forest King system.
Frustrated with the lack of movement on that front, I finally decided to do something very ambitious, very exciting, very mid-life-crisisy. Aided only by everyone I had worked with, was related to or had ever met, I single-handedly created this unique little epic. A supervillain musical, of which, as we all know, there are far too few.
The idea was to make it on the fly, on the cheap – but to make it. To turn out a really thrilling, professionalish piece of entertainment specifically for the internet. To show how much could be done with very little. To show the world there is another way. To give the public (and in particular you guys) something for all your support and patience. And to make a lot of silly jokes. Actually, that sentence probably should have come first.
The program of events may be found here.
And be sure to pass this news along and get yourself a banner for your own site... It will be epic! LOL!