Forget online drives, sync directly with BitTorrent Sync

New BitTorrent Sync offers decentralized file syncing between your computers.

If you do not trust online storage drives for file syncing across your devices or are frustrated with storage limits, there is another player in town. BitTorrent has released a new alpha version of its Sync software, which supports syncing folders across the Internet without going through an intermediary like Dropbox, Cloud Drive, or iCloud.
The Sync software uses the standard BitTorrent decentralized file-sharing technology to establish a secure file-synchronizing routine between your various devices. It does so by use of local peer discovery, peer exchange, and static known hosts as well as DHT and classic BitTorrent trackers to establish links between your systems, and then uses standard P2P BitTorrent protocols to transfer and reassemble files in chunks.

Right-click a shared folder to copy the folder's shared secret encryption key for transfer to another system.

Files are sent directly between your computers if possible, but if not then relay servers are used. All traffic is encrypted with 256-bit AES encryption, and the use of an encryption "secret" prevents data from being read by anything but your authorized devices.
After installing the software, you select a folder or two to synchronize, and then generate a 21-byte encryption secret to link up your various devices. Copying this secret to the BitTorrent Sync client on another computer will establish the appropriate link and allow file sync to start immediately.
The benefits of this technology are that you do not rely on a third-party intermediary as a synchronization medium, and as such the only limit to storage size is the capacity of your systems' hard drives (and perhaps bandwidth limits imposed by your ISP).

With the shared secret copied, pasting it to another system's BitTorrent Sync settings will start the encrypted sync. Be sure to keep your 'secret' truly secret.

When initially announced in January, BitTorrent Sync was kept as a private test, but BitTorrent has made the latest release of Sync available to the public on Windows (XP SP3 or later), OS X (10.6 Snow Leopard or later), and Linux (kernel 2.6.16 or later) platforms. The latest version has added one-way synchronization options, and settings to exclude subdirectories and specific files.

Currently the sync does not offer versioning and backup management, but if your systems are coupled with Time Machine or another backup routine then files should be properly secured.
While it's available to the public, do keep in mind that BitTorrent Sync is still in alpha development, meaning that it will likely undergo significant changes in the upcoming months, and potentially have serious bugs in its code that could lead to data loss; however, it may be fun to try for those who regularly access multiple systems and wish to keep certain directories of files up to date.