Hatsune Miku, Vocaloid: Computer Generated Virtual Singing Doll
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.
Here's her concert, in LA, during Anime Expo 2011.
Hatsune Miku Singing in English
LOL. At least she's in tune.
Her singing doesn't sound that good now, but it'll probably beat any human soon.
Hatsune Miku Singing Elfen Lied
Here's Miku sining 〈Lilum〉, a song in Latin.
Compare the singing to real humans, see: Kumiko Noma - Lilium (Elfen Lied).
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.