CloudStore

CloudStore synchronizes files (and directories) between multiple computers. It is fast, reliable and extremely secure.

And of course, it is free software (LGPL)!

Subshare

Subshare is CloudStore on steroids: It also synchronizes files (and directories), but it encrypts all data, before they leave the client-computer. This makes it possible to host your data on an untrusted server and still be safe from the NSA & other enemies of civil liberties. You can also share your data with friends or colleagues; well-protected by your OpenPGP-web-of-trust.

Cumulus4j

Cumulus4j was originally developed by NightLabs. But since NightLabs discontinued this project and its authors work with CodeWizards, now, Cumulus4j is currently in the process of becoming an official CodeWizards product.