We just released sabre/dav 3.0
The fruux syncing system is fully based on open standards, and our core is also "open sourced". In a nutshell, this means we give it out for free to other developers, so they can take advantage of our innovations and in some cases contribute back to the community.
The name of this project is sabre/dav. This project is now primarily funded and developed here at fruux. The project has been active for nearly 8 years, and today got it's third major release.