[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.
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.
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
DiskCryptor is a free and open-source utility that offers encryption of all disk partitions, including the system partition. Originally conceived as a replacement for DriveCrypt Plus Pack and PGP Whole Disk Encryption (WDE), the project aims to create the best product in its...
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.
The Gnome Encfs Manager is an easy to use manager and mounter for encfs stashes. It allows you to create encrypted directories, any file that is placed in such a directory will be encrypted.
FreeOTFE is a free, open source, "on-the-fly" (OTFE) disk encryption program for PCs running MS Windows and Windows Mobile PDAs (FreeOTFE4PDA).Discontinued
Discontinued per Wikipedia page (https://en.wikipedia.org/wiki/FreeOTFE)
Data encryption software by CryptoForge allows you to secure your sensitive data everywhere it goes with strong AES encryption. It features a file encryption module which allows you to encrypt files and folders of any type and size, on any kind of media, including external USB...