Interesting for people that do not trust Cloud Storage providers: BitTorrent Labs’ BitTorrent Sync.
I’ve tried an early version, and it works “OK”. Not yet as well-integrated like for instance DropBox, but stable and fast enough.
Important thing for me: works on *nix, OS X and Windows. Hopefully mobile devices will follow soon.
At the time of writing (May 2013), it is the only cross platform freeware entry in Comparison of file synchronization software – Wikipedia, the free encyclopedia.
In the mean time, Thomas Mueller (dummzeuch)wrote a nice blog post with background information on this: Bittorrent Sync, a secure DropBox alternative » twm’s blog.