Free open-source disk encryption software for Windows XP/2000/2003 and Linux
Main Features:

* It can create a virtual encrypted disk within a file and mount it as a real disk.

* It can encrypt an entire hard disk partition or a device, such as USB memory stick, floppy disk, etc.

* Provides two levels of plausible deniability, in case an adversary forces you to reveal the password:

1) Hidden volume (more information may be found here).

2) No TrueCrypt volume can be identified (TrueCrypt volumes cannot be distinguished from random data).

* Encryption algorithms: AES-256, Blowfish (448-bit key), CAST5, Serpent (256-bit key), Triple DES, and Twofish (256-bit key). Supports cascading (e.g., AES-Twofish-Serpent).

* Based on Encryption for the Masses (E4M) 2.02a, which was conceived in 1997.

View: TrueCrypt Documentation | Change Log

Screenshots: TrueCrypt In Action

Download: Latest Stable Version - 4.0

I don't use this software, so I have no idea about it. I found this on the main webpage.