Hatsune Miku, Vocaloid: Computer Generated Virtual Singing Doll
Hatsune Miku (初音ミク) is a 16-years-old girl character representing a singing vocal synthesizer Vocaloid, developed by Yamaha.
Vocal synthesizer is a machine that generate singing human voice. So, it'll sing any song, eventually, as perfect or better than human singers.
Hatsune Miku Concert in Tokyo
Here's her concert, in Tokyo, Japan.
Hatsune Miku Singing in English
LOL. At least she's in tune.
Her singing doesn't sound good now, but it'll probably beat any human soon.
Hatsune Miku Singing Elfen Lied
See also: Elfen Lied - “Lilium”
There's sci-fi anime story Macross Plus, where a computer AI produced female entertainer named Sharon Apple, represented as a hologram, became a galactic pop idol. Miku Hatsune is a real-life version of Sharon Apple. See: Sharon Apple - Information High (Macross Plus).
Hatsune Miku in Second Life
for more about Second Life, see: Second Life Screenshot Gallery.
Thanks to Gwern Branwen, wizardgynoid for tips.