Wait, no, I mean Dr. Horrible's Sing-Along Blog.
You can find the details (as un-detailed as they are) here at the Dr. Horrible Master Plan Page. Basically, what we have here is Joss Whedon + Nathan Fillion + Neil Patrick Harris + songs + comic books -- in case you're having trouble carrying the "i's" that equals a supervillain musical.
Act I went up just a little while ago (why no, I did not stay up late waiting for it, that would be silly), and I think it's hilarious. Acts II and III will be appearing over the course of the week, and then vanishing into the ether come the weekend. At this point, of course, we will be expected to shell out large amounts of cash for a copy of Dr. Horrible's Sing-Along Blog.
Which we will gladly do.
[After you've watched Act I, come back here so we can talk about how fantastically dickish the "super hero" is. I want his shirt! As in, the actual shirt that he is currently wearing, because he's hot, and .... um, I'll just leave now. Go watch the clip.]