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