TrueCrypt

Protect your sensitive data with this free open-source disk encryption software. Creates a virtual encrypted disk within a file and mount it as a real disk. Encrypt an entire hard disk partition or a device, such as USB flash drive. Automatically and transparently encrypt in real time.

TrueCrypt provides two levels of plausible deniability, in case an adversary forces you to reveal the password: a hidden volume (steganography- more information may be found here); no TrueCrypt volume can be identified (volumes cannot be distinguished from random data). TrueCrypt uses encryption algorithms AES-256, Blowfish (448-bit key), CAST5, Serpent, Triple DES, and Twofish. It is based on Encryption for the Masses (E4M) 2.02a, conceived in 1997.