I've played around with a couple of van de Graff generators here and there, but the Theater of Electricity at MIT is way more impressive than anything I've seen in person.
Their van de Graff generators are like twenty feet high. The picture that I've included shows one of the demonstration scientists in a cage being fried with a really big frickin' spark. From what I can tell, the Theater of Electricity is a part of the Museum of Science. Now, how did I miss that back when we were in Boston a couple of summers ago?