It seems like every day there is a new cloud service that lets you store and sync your data. They all offer varying levels of encryption, security and privacy and are all priced relatively similar. Most users are perfectly fine with the limitations that come with their cloud service of choice and many are happy to use the free edition or pay a fee for the convenience of it.
I've never really been a big fan of most of the cloud providers, whether it was due to pricing or giving a third party access to all of my data. There are relatively few providers that offer true, trust no one (TNO), levels of security. Most providers use some form of encryption during upload and storage, but most of the time they can usually decrypt it when compelled without any form of action from you. Even if it's just to build a convenient search index or rendering online previews, your files are being opened and read in some form or another.
There are many users, for the reasons previously listed or for reasons of their own, that would prefer the convenience of cloud syncing without the cost or worry over who has access to what. There are already some really powerful tools that have been around for a long time that can make this a reality. Rsync is a fast incremental file transfer tool that's very common in the Linux and server world. In the past I've used rsync on my computer along with an Android client to wirelessly copy everything stored on my phone. It worked, but it was far from the best experience. I wanted to sync my entire data directory (emulated sd card storage on my Galaxy Nexus and newer phones without sd cards), but due to the large number of files and the sheer file size, the rsync Android client would often choke and just stop working. It wasn't a huge issue since rsync is incremental, I could just restart it and go from there, but it was far from optimal.
Enter BitTorrent Sync. Most people are familiar with BitTorrent used for P2P downloading, legal and illegal file sharing. BitTorrent Sync takes the phenomenal protocol that is BitTorrent and uses it to sync folders and data between your devices without any need for a centralized server. BitTorrent Sync supports read-only along with read/write access making it super easy to let your friends and family access photos and other data by copying and pasting a key from their email into a BitTorrent Sync client or scanning a QR code with their phone without having to worry that they might accidentally delete your only copy. BitTorrent Sync also excels at large media files, so you'll never need to worry about emailing a video or uploading to YouTube.
Syncing could be a hassle if your devices are not online at the same time, but this is an easily solvable problem. For those familiar with git and other DVCSs, you can easily push and pull code directly from other peers, just like you can directly sync data between peers with BitTorrent Sync, but for the most part everyone still seems to use a central server (ala GitHub) because it's easier and there are lots of great features that come along with it. To solve the issue of offline devices an always-on server can be used. There are numerous tutorials for using BitTorrent Sync with Amazon's AWS or other Linux servers, but it really shines if you already have an external hard drive or NAS and a Raspberry Pi or similar low power computer. Jack Minardi has written a very good tutorial on BitTorrent Sync and a Raspberry Pi that steps you through everything you need to get going (UPDATE: v1.4 was recently release with some new features and changes, make sure to download the most recent Linux ARM copy). If you do go this route, make sure to go through Jack's Extra Credit section for starting on boot and securing access to your web based control panel. In preparation for setting up BitTorrent Sync, I also used a script to remove a lot of the unneeded packages that come installed on the Raspberry Pi's default OS when you're going to run it as a headless server. My fork can be found on GitHub
If you're hoping for browser based previews and access to your files, you may want to also checkout OwnCloud which has a bunch of additional private cloud features.
Give BitTorrent Sync a try and see what you think, I'm interested in hearing what uses others can dream up for it. I'm already using it to backup my entire phone (apps backed up with Titanium Backup to storage and storage is backed up wirelessly to my external hard drive). In the future I'm thinking about using it to transparently share my camera folder with others.