I came across this yesterday and it is pretty cool. To be honest I was never into Buffy the Vampire Slayer and I don’t think I have ever seen Angel or Firefly. I did know they were the brainchild of Joss Whedon though and so is Dr. Horrible’s Sing-Along Blog. Dr. Horrible’s Sing-Along Blog will have three episodes of about 15 minutes and you can catch them all for free for a limited time only so I recommend getting over there now. The second act is already online. After that you will be able to purchase it from the likes of iTunes and there is rumor of a DVD version in the future. It stars Neil Patrick Harris (How I met your Mother) as Dr. Horrible and a few other recognizable faces. The production values are high and I highly recommend it. How often do you get to see a super-villain musical.
Oh, and there are t-shirts but they are pretty horrible. Available from Jinx.