Vocaloid is a Japanese synthesized voice system that you can buy and all of their different systems and make songs.
Different (Japanese) systems:
Megurine Luka (English as well...)
And there are also English and fan made ones as well.
If people edit Vocaloid voices and make their own characters to them they can also be accepted into a thing called UTAU (UTAUloid I've also heard it called) which is like a completely fan made group. ((The fan made things started mainly on nicovideo.jp))
My favorites would have to be Rin and Len. ^^ Rin is so cute and has an amazing voice and Len is entertaining and is abuse bait. I like their songs Daughter of Evil and Servent of Evil and all of the Kokoro ones as well.