Vocaloid is a voice synthesizer program for Windows computers made by Yamaha.  Though Yamaha created the software, the Vocal Libraries were created by third party developers under contract.

Vocaloid comes in two versions, the second being more expansive and customizable than version one, and is reverse compatible.  The interface comes in English or Japanese language, and any vocal library can be installed on top of it.

Vocaloid was designed to create music, and many users post their productions on Japanese video-sharing site, NicoNicoDouga. Each Vocaloid library sells for around $150 US.  There are other ways to pirate the software, but we all want to keep things legal, so I'm not saying a word.