YouTube: Vocaloid Hatsune Miku, the worlds virtual diva
http://www.youtube.com/watch?v=xBZOlipfjkQ
It's interesting to see how incredibly popular this has gotten since it's release in 2007. For those unfamiliar with Vocaloid (wiki), it's an immensely popular sining synthesizer software that allows people to treat voice(s) as (an) instrument(s). To give some perspective, at least one CD published using Vocaloid vocals has hit the top 10 charts in Japan and some songs are available via amazon.com and itunes (link).
Some extremely popular examples:
Warning!
Extremely high-pitch will likely irritate people. I highly recommend you start at low volume, then increase to taste. Also, niconico (ie. Japanese YouTube equivalent with comment overlay) is where most of the videos are, requires complicated registration, and generates many many more "views" than youtube.
Melt
http://www.youtube.com/watch?v=m4jkxXyCBgs
The World is Mine
http://www.youtube.com/watch?v=MDWf-pPYNCw
http://www.youtube.com/watch?v=Qe_SDakpPx4 <-- Subbed, different video
Black Rock Shooter
http://www.youtube.com/watch?v=YyB4gLFF-Xw
Others
http://www.youtube.com/watch?v=8kcpW0HmIbg <-- Get Away / Chicago (English)
http://www.youtube.com/watch?v=bAu0T2dIc84 <-- Easy on the ears, Reality
http://www.youtube.com/watch?v=WutJPH9_ZJo <-- Tori no Uta (Air Opening)
http://www.youtube.com/watch?v=kbbA9BhCTko <-- "Ievan Polkka"
http://www.youtube.com/watch?v=HATpOha7DFg <-- Belinda Carlisle's "Heaven Is A Place On Earth" (English)
http://www.youtube.com/watch?v=evRPBf531IU <-- Laputa Ending, Carrying You
http://www.youtube.com/watch?v=qvRqnd4ymus <-- Still Alive (Japanese)