Hatsune Miku live :: kinda epic for a piece of software
It's the fans going mad down the front that makes it for me... that, and the fact she's a voice synthesizer.
http://en.wikipedia.org/wiki/Hatsune_Miku
"Hatsune Miku (初音ミク?) is a singing synthesizer application and its female character, developed by Crypton Future Media. It uses Yamaha Corporation's Vocaloid synthesizing technology. Her voice is sampled from Japanese voice actress Saki Fujita."