In the words of my eldest brother,

"Run, do not walk to the iTunes store and spend the $3.99 for the season pass to [Dr. Horrible's Sing-along Blog]. ...End of Line... Seriously, do it now."

I know what you're thinking, I thought it too. How could anything be so good as to elicit this type of ultimatum from a brother to a sister. But after having done his bidding and watching it, I have only this to add: GO NOW. You MUST see this, it is going to be the best thing you've watched in a long time. Don't even talk to me until you done this. Seriously.