If you've watched Futurama at all, then you'll get the joke. For those who haven't, the show has a running gag involving the heads of famous people - ranging from Jonathan Frakes to Richard Nixon - are kept preserved and alive in jars like the above.
I like how they put Dr. Horrible and Doogie Howser right next to each other. I think they used too many Doctors from Doctor Who, though. Still, it's super cool and also available as a shirt.