From their GitHub repo page:
FontForge is a free (libre) font editor for Windows, Mac OS X and GNU+Linux. Use it to create, edit and convert fonts in OpenType, TrueType, UFO, CID-keyed, Multiple Master, and many other formats.
FontForge also has a book about how to design typefaces through its software (hosted on GitHub Pages, no HTTPS connection).
Thanks for sharing this! Very helpful in a work project I’m just starting
Glad to see someone finding this useful as much as I did!