UTAU

UTAU is a vocal synthesis program made by Japanese developer Ameya. It allows the user to create their own voicebanks, and it also allows the use of different synthesis engines, as well as the two included by default. UTAU is very popular due to being "free" and allowing users to create and share their own voicebanks for the program.

Resamplers
Main article: UTAU/Resamplers

UTAU allows the use of different synthesis engines—or resamplers—which work in different ways, with some having more features than others, having a different overall sound to them, or working better with different voicebanks.