As far as I know the current contenders are git-annex[1], syncthing[2] and clearskies[3]. They're all still very green. git-annex is technologically farthest along but is still very unpolished for the basic "sync this dir" functionality to be reliable/friendly. Clearskies is still at proof-of-concept stage, defining the protocol and implementing the first clients. syncthing is probably the best bet right now, but I haven't tested it yet.

[1] http://git-annex.branchable.com/

[2] https://github.com/calmh/syncthing

[3] https://github.com/jewel/clearskies-ruby