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.
Here's her concert, in Tokyo, Japan.
Following is a video (13 min) in Japanese introducing her and the technology. You get to see Japanese people, city, Miku's singing, 3D modeling technology, and fandom.
LOL. At least she's in tune.
Her singing doesn't sound that good now, but it'll probably beat any human soon.
Here's Miku sining 〈Lilum〉, a song in Latin.
Compare the singing to real humans, see: Kumiko Noma - Lilium (Elfen Lied).
Hatsune Miku reminds me of a sci-fi anime story Macross Plus, where a computer AI produced female entertainer, represented as a hologram, became a galactic pop idol. See: Sharon Apple - Information High (Macross Plus).
for more about Second Life, see: Second Life Screenshot Gallery.
Thanks to Gwern Branwen, wizardgynoid for tips.