[TrueCrypt is not exactly "Open Source", but the source is available. See the complete description and the comments to get more informations.]
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.
Though it has not been audited yet, it has many features and is one of the most popular encryption solutions: More Info »
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).
Encryption is automatic, real-time (on-the-fly) and transparent.
Parallelization and pipelining allow data to be read and written as fast as if the drive was not encrypted.
Encryption can be hardware-accelerated on modern processors.
Provides plausible deniability, in case an adversary forces you to reveal the password:
Hidden volume (steganography) and hidden operating system.
For the portable mode you need admin rights, see http://www.truecrypt.org/docs/?s=truecrypt-portable
Is TrueCrypt "Open Source"?
While TrueCrypt is source-public, it is a misconception that it is open source or free software. It is not considered Open Source as defined by the Open Source Initiative or free software as defined by the Free Software Foundation and no major GNU/Linux distribution (Arch, Debian, Ubuntu, Fedora, openSUSE, Gentoo) has accepted it because of license restrictions that are incompatible with being free/libre and open source.
http://en.wikipedia.org/wiki/TrueCrypt#Licensing_and_Open_Source_status More Info »