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

TrueCrypt is a software system for establishing and maintaining an on-the-fly-encrypted volume (data storage device). On-the-fly encryption means that data are automatically encrypted or decrypted right before they are loaded or saved, without any user intervention. No data stored on an encrypted volume can be read (decrypted) without using the correct password/keyfile(s) or correct encryption keys. Entire file system is encrypted (e.g.., file names, folder names, contents of every file, free space, meta data, etc).

Main Features:
Creates a virtual encrypted disk within a file and mounts it as a real disk.
Encrypts an entire hard disk partition or a storage device such as USB flash drive.
Encryption is automatic, real-time (on-the-fly) and transparent.
Provides two levels of plausible deniability, in case an adversary forces you to reveal the password:

1) Hidden volume (steganography â?? more information may be found here).
2) No TrueCrypt volume can be identified (volumes cannot be distinguished from random data).
Encryption algorithms: AES-256, Serpent, and Twofish. Mode of operation: LRW.