There are multiple free software singing synthesizers, QTAU is one of them. It started as a clone of UTAU but now has many uniqe features.
QTAU is a multi-track piano roll editor for GNU/Linux. It is designed for editing lyrics for singing and supports MBROLA, WORLD, eSpeak NG, and Festival engines for synthesis. Since it shares WORLD with vConnect-STAND and OpenUTAU it is also compatible with UTAU.
QTAU comes with its own synthesis engine called VoSamp which not only supports UTAU voices, but also experimental MBROLA and Festival voices with the eSpeak NG frontend.