Of course, you can watch it for free online if you watch some brief commercials... but the online version doesn't have the commentary, aka "Commentary! The Musical" which Joss Whedon describes thusly:
"Commentary! The Musical" is the most painstaking and exhausting piece of whimsey I have ever mistaken for a good idea. It has nearly twice as much music as Dr. Horrible itself -- since you can't really talk that much during a commentary musical or it sounds like a regular commentary. (Which we also have, with the stars and writers, plus making-of's, ELE applications, and a few items left lying around by a notorious Bunny...) I can say without hesitation that I hesitate to say it's great. And by great I mean ridiculous. It's sophmoric, solopsistic, silly and the most fun I've had being exhausted since the fabled Mushortio itself. And everyone sings beautifullly. Which enrages me. I a little bit hate my friends now.
As for the show itself, my comments are as follows: A) it's just as silly as you'd expect from the title; B) my wife and I have been singing it at each other for months; C) it's probably not for everyone; D) I'll be preordering a DVD post-haste.
Ok, announcement over. As you were.