T r u e C r y p t is free open-source disk encryption software for Windows 7/Vista/XP, Mac OS X, and Linux.
It encrypts an entire partition or storage device such as USB flash drive or hard drive.
It also 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.
It provides plausible deniability, in case an adversary forces you to reveal the password: hidden volume (steganography) and hidden operating system.
It's a good, simple-to-use encryption tool, but now it is replaced by VeraCrypt, which supports TrueCrypt vaults and will convert TrueCrypt to VeraCrypt format if required.