Vocaloid is a singing synthesizer application software developed by the Yamaha Corporation that enables users to synthesize singing by typing in lyrics and melody.
^Stolen from Wikipedia^
Anyway, here are some vocaloid examples.
Love is war - Singer: Hatsune Miku
The world is mine - Singer: Hatsune Miku
http://www.youtube.com/watch?v=Qe_SDakpPx4Servant of Evil - Singer: Kagamine Len
I like you I love you - Singer: Kagamine Rin
The never ending story - Singers Megurine Luka and Hatsune Miku
http://www.youtube.com/watch?v=TaSZ0siQjXADouble Lariat - Singer: Megurine Luka
http://www.youtube.com/watch?v=rXNH8TEP8vMThe disappearance of Hatsune Miku - Singer: Hatsune Miku (It's a sad song T_T)
http://www.youtube.com/watch?v=_xtEy951DicRotten girl, grotesque stalker - Singer: Hatsune Miku
http://www.youtube.com/watch?v=xGThHuZ3lBQUninstall - Singer: Kaito
http://www.youtube.com/watch?v=k0uU2PZVfM8...feature=relatedDark Woods Circus - Singer: Various
http://www.youtube.com/watch?v=AU6PL0LH4_YMagnet (LIVE CONCERT) - Singers: Luka and Miku
http://www.youtube.com/watch?v=slM1T6HF5jwTrick or Treat - Singers Rin and Len
http://www.youtube.com/watch?v=Pb6fSiMBbv4And there are a LOT more.