These vocalizers are made by the Yamaha Cooperation, and first published 2003. With that program, you type into the melody and lyrics, and also add effects, and voila, you have a song sung by a synthesized voice. Sounds easier than it is, for sure. For the making of these voices, real singers and artists have been used. Many of these voices have an Avatar and Name. What at first was known in Japan only, now is getting known worldwide as a new type or possibility for music.
As I don't have this program, and can't tell you more about it, if you're interested in it, you should visit the Vocaloid Website.
Now, the most famous of these Vocaloids is undoubtly Miku Hatsune who was made 2007. Her voice has been done through Saki Fujita. She even gives live concerts already! These work because of holographic technology, where her Avater is imaged on a big glass screen. Here's an example:
This concert has been done by Project Diva and SEGA. Like this, you can see that the actual song is still performed by a band, while the lyrics are by the Vocaloid!
Well, there is also a famous Japanese Star that lend his voice for this Vocaloid project, Gackt. This Vocaloid is called Gackpoid or Kamui Gakupo. So here is an example on how the voice from a star sounds like when being synthezised.....:
But my favorites stay the "Tragic twin couple" Kagamine Rin and Kagamine Len, also created 2007, and whose voices are by Asami Shimoda. Why twins? Well, they look like they are, but they are not! And why tragic? Well, because in many songs in which they sing together, one of them usually dies.
But here is a rather cheerful song by only Kagamine Rin. I chose this because....well, sometimes I'm like that, ahahaha:
Well, and if you like it and want to hear more, you can find many of these songs and videos on Nico Nico Douga.