[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.
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.
EncFS provides an encrypted filesystem in user-space. It runs without any special permissions and uses the FUSE library and Linux kernel module to provide the filesystem interface. You can find links to source and binary releases below.
The author has agreed that EncFS in its current form is dangerous (to security) for several reasons. Wikipedia has a brief summary: https://en.wikipedia.org/wiki/EncFS#General_Security_ConcernsThe actual audit, which the author participated in, is here: https://defuse.ca/audits/encfs.htm
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.