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.

In version 5.0 TrueCrypt now offers the ability to encrypt a system partition/drive (i.e. a partition/drive where Windows is installed) with pre-boot authentication (anyone who wants to gain access and use the system, read and write files, etc., needs to enter the correct password each time before the system starts).

Get it here: Truecrypt