[TrueCrypt is not exactly "Open Source", but the source is available. See the reviews to get more information.] TrueCrypt is a disk encryption software for Windows Vista/XP, Mac OS X, and Linux. Creates a virtual encrypted disk within a file and mounts it as a real disk. Encrypts an entire partition or storage device such as USB flash drive or hard drive. Encrypts a partition or drive where Windows is installed (pre-boot authentication).
Currently unmaintained. Last version, TrueCrypt 7.1a, is still available for download and development has continued via the forks https://alternativeto.net/software/veracrypt/ and https://alternativeto.net/software/ciphershed/. "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."