Dr. Horrible's Sing-Along Blog. Go see it while it's still up and free. The FAQ says it goes down at midnight Sunday, July 20th. (I don't know if they're using that correctly to mean tonight, or to mean tomorrow night.)
If you've managed not to hear about it yet, it's about 40 minutes of musical about Dr. Horrible, aspiring supervillain, written/produced/directed by Joss Whedon, starring Neil Patrick Harris, Nathan Fillion, and Felicia Day. It's free at the website in three acts, then it will be available through iTunes and on DVD.
I thought it was pretty good. I'm not convinced it was the best thing ever. But you should definitely see it. For two reasons: One, it's entertaining enough to be worth 40 minutes of your life. Two, you're going to spend at least the next month pissed off at all of your friends as they quote it incessantly if you haven't seen it to understand the references. 'cause I'm certain we'll be hearing, "The hammer is my penis," for a long time to come.