Crypter!

[TrueCrypt is not exactly "Open Source", but the source is available. See the reviews to get more information.]

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.

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. A wealth of frontend applications and libraries are available. Version 2 of GnuPG also provides support for S/MIME.

AxCrypt is a simple file encryption software for Windows. It integrates seamlessly with your workflow to encrypt, decrypt, store, send and work with files.

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). A Java library is also available for developers using Java to read and write AES formatted files.

No encryption of dynamic disks, but command-line-encryption! Dynamic disks must be converted to basic disks with EasyUS Partition Master or Partition Guru before encrypting with Bitlocker. Windows BitLocker & "Bitlocker To Go" Drive Encryption is a data protection feature in Windows 10 Professional, Windows Vista Enterprise and Windows Vista Ultimate for client computers and in Windows Server 2008. BitLocker protects against data theft or exposure on computers that are lost or stolen, and offers more secure data deletion when computers are decommissioned. Bitlocker To Go (BTG) is essentially Bitlocker for external drives. It’s full volume encryption for all your USB drives: http://blogs.msdn.com/b/rockyh/archive/2009/03/01/bitlocker-to-go.aspx »

DiskCryptor is the only truly free solution, provided under GNU General Public License (GPLv3), which offers encryption of all disk partitions, including the system partition. The main criteria for open-source software is the availability of its source code under one of the open source licenses. The only open-source alternative to DiskCryptor that has comparable features is TrueCrypt. However, because of the restrictive license under which TrueCrypt is provided — the TrueCrypt Collective License — TrueCrypt cannot be classified as a truly free software, as it places limits on the use and modification of its source code by developers. There are other alternatives with similar functionality, but they are fully proprietary ones, which makes them unacceptable to use for protection of confidential data. »

GPG for Mailis an open source plugin for Apple Mail. Encrypt, decrypt, sign and verify mails using OpenPGP with a few simple clicks.

If your computer were ever to be stolen, are you certain that your personal information cannot be discovered? Jetico’s BestCrypt software automatically encrypts your files so that nobody can read the data unless they have the right password or keys.

This is an experimental project of porting encfs to the Windows world.

DCrypt is an encryption and decryption tool that combines in a smart way multiple algorithms. The combination of multiple strong algorithms makes it much more difficult for hackers to get the plain content of the encrypted file without the password.

LUKSUS is a tool that creates an encrypted volume and filesystem on a hardrive or other storage media as well as a filecontainer.It can use the following encryption facilities: LUKS, Truecrypt and GELI.

SecurStick is a free portable application that lets you carry encrypted files on your USB drive. You can use the drive on any computer to access and modify those files WITHOUT needing any administrator privileges.

LUKS Manager provides on-the-fly encryption (AES by default) to virtual folders on Android devices.