UTAU

From Vocaloidism Wiki

UTAU is a spin-off program that is not as good as VOCALOID. It is free and downloadable, however only works on japanese activated computers. It will not work properly if it isn't a japanese related computer. UTAU is also very difficult to use. You can also create your own UTAU.

Some of the famous UTAUs are Kasane Teto and Yokune Ruko. There are many more UTAUs. However, these are the ones who pretended to be VOCALOIDs, then UTAU adopted them.