St. Cecilia Press houses the transcriptions of Renaissance and Medieval music made by Monique Rio as well as the singable translations of Carol Anne Perry Lagemann.
Monique's transcriptions are done in Lilypond, and may be used with attribution. (CC By 4.0)
All collections can be found on Github.
If you want to help with their transcription projects or let them know about errors in the music, send them a message.