UTAUloids are, you can say, the first project that launched voice synthesizer programs. There are so many since they are all free and many are fan-made. There were some that were more well known than others though. Since there are over 150 if them, I will not name them all. Go check out UTAUloids wiki if you have an interest in finding out about them.
The most famous Japanese UTAUloids~
'Defoko' Uta Utane
There aren't much original UTAU songs unlike the Vocaloids. They mainly sing Vocaloid songs or anime songs. And, they're all free.
There also a special brand of UTAUloids called Maidloids since their character designs are all 'maid style' clothes.
Nana Macne Petit
They are more for voice acting than singing though and many people use them to make 'echi' songs since they are maids. *cough cough* Maidloids are the only ones you have to pay for, apparently.
Favorite UTAUloid songs:
My favorite of all the UTAUloids just has to be Teto I mean, she is awesome! Teto FTW!!
She is adorable and cute. And her voice is amazing!! We love you Teto!! <3