Seafile

Please note:

1. Seafile was a company with two branches - one in Germany (Seafile.de) and one in China (Seafile.com). In the summer of 2016, the two parted company.
2. Since then until now (March 2017), Seafile.de continued to provide a service syncing people's files using the open source Seafile client. Very good prices, very nice service, and an excellent client - one of the very few I've found able to deal with (literally) hundreds of thousands of small data files without breaking down. Great upload/download speeds. (I live in the EU, but not in Germany.)
3. As of March 2017 I noticed a rebranding of Seafile.de to Syncwerk.com (they Seafile.de redirects there now). I think Seafile GmbH (the German company) are seemingly going for a rebrand to move towards a new future. Good luck to them. I'm on board as a paying customer. Let's see where it leads. I hope they maintain the open source nature of their client.

If you don't know: Seafile.de (now SyncWerk.com) are a very reliable, low-price alternative for file syncing of the Dropbox type. Additionally, their client allows encryptions of the files you store. And the files are stored on EU territory. All of that is hugely important. By contrast, Dropbox are closed source, unencrypted (they encrypt only server-side, which is to say they can read your data) and they are US-based, which means they must hand over data at the NSA's behest without even telling you (see National Security Letter). So go with Seafile/SyncWerk. Or Tresorit. Or SpiderOak. But at least someone who treats your data with the decency and privacy you deserve.

I might be wrong here, but it seems that, originally, Seafile was an company that provided both the open source software called Seafile (a server software and the client that talks to it to sync files), and also it offered a service for a fee that used this software to store files on their server. The total package was therefore something like Dropbox, but with an open source server/client software. The company had a German and a Chinese branch.

In the summer of 2016, the German and Chinese parts somehow disagreed with each other. They went their separate ways. The Chinese arm (Seafile.com) still offers downloads of Seafile the software, different versions at different prices. The German arm (Seafile.de) offers a sync service (like Dropbox) that uses this software (at least the client), which you can download from them when you sign up. They have promised to fork the software and develop it independently.

Seafile.de is an excellent, fast and versatile Dropbox alternative. Your files will be stored under German jurisdiction. At the time of writing 500GB cost only 5 Euro (approx. $5 USD). The client software is very versatile, allowing you to sync multiple different folders and share them with other Seafile.de users.

Use it with Cryptomator to get end-to-end encryption and therefore additional privacy.

Seafile should be congratulated for offering optional client-side encryption of a folder you might want to sync. This is good because the files are encrypted on your computer before they are uploaded, which means that they are not readable to Seafile or anyone who might access their servers (legally or illegally). This makes your data more secure and private than you get with Dropbox, GoogleDrive, Box or OneDrive.

However...

You should know that the encryption the Seafile software provides encrypts the file contents only and not the name, file size, folder structure or the date any particular file was modified. This is a limitation if you want to prevent anyone learning what files you have, even if they can't read the contents.

Therefore...

A good solution is to pair Seafile's service with an open-source encryption program like Cryptomator. The reason for this is that Cryptomator provides a very simple way to encrypt your data on your computer before it syncs, but it does encrypt the file names, folder structure and other metadata. Cryptomator is also preferable for this purpose over competitors like BoxCryptor because it is free and open source, and therefore its code can be inspected to ensure that the encryption is implemented correctly.

Great alternative to Dropbox! Why not in standart Linux repos?

Very clean, good desktop integration, free and open source (self-hosted).