Cryptomator provides transparent, client-side encryption for your cloud. Protect your documents from unauthorized access. Cryptomator is free and open source software, so you can rest assured there are no backdoors.
[TrueCrypt is not exactly "Open Source", but the source is available. See the reviews to get more information.]Discontinued
The development of TrueCrypt was ended in 5/2014 after Microsoft terminated support of Windows XP. Windows 8/7/Vista and later offer integrated support for encrypted disks and virtual disk images. Such integrated support is also available on other platforms. You should migrate any data encrypted by TrueCrypt to encrypted disks or virtual disk images supported on your platform.
GnuPG allows to encrypt and sign your data and communication, features a versatile key management system as well as access modules for all kinds of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications.
BitLocker Drive Encryption is a data protection feature that integrates with the operating system and addresses the threats of data theft or exposure from lost, stolen, or inappropriately decommissioned computers.
AES Crypt is an advanced file encryption utility that integrates with the Windows shell or runs from the Linux command prompt to provide a simple, yet powerful, tool for encrypting files using the Advanced Encryption Standard (AES).
Encrypto lets you encrypt files before sending them to friends or coworkers. Just drop a file into Encrypto, set a password, and then send it however you normally would — but this time with added security. It’s free for both Mac and Windows.
AxCrypt is a simple file encryption software for Windows. It integrates seamlessly with your workflow to encrypt, decrypt, store, send and work with files.
Boxcryptor is an easy-to-use encryption software optimized for the cloud. It allows the secure use of cloud storage services without sacrificing comfort.
CryptSync is a small utility that synchronizes two folders while encrypting the contents in one folder. That means one of the two folders has all files unencrypted (the files you work with) and the other folder has all the files encrypted.