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

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.

http://www.truecrypt.org/

Any MacOS X people been able to use this?