There are many good open source projects alternative to Cloud-System services that provide File Synchronization support. The most famous I've seen so far are Syncany and sparkleshare, but I'm pretty sure that more projects are coming out. When I started working on Quambla it was Feb 2011 and no one of these project was out yet.
The risk for all these projects is always the same: they can get discontinued at anytime, because the people that invented them do other jobs. In a similar scenario only worthy projects will survive, if they will somehow capture the interest of someone willing to join them.
The last version of Quambla is yet a prototype than a stable application, there are still lot of things to fix, the GUI is too rough and does not to let the user retrieving older revision of the files, feature that is already implemented.
I am working on two different versions of Quambla: one with a DB support and one without it.
The first provides better performances and it is more stable, but you should need to have a remote accessible mysql server.
The second (ftp-olny version) in trying to bypass a DB but performances are degrading quickly.
Please, join the project or if you intend to help the project and make Quambla growing up.