TrueCrypt, An Encrypting tool

This tool is very good,

"T r u e C r y p t
Free open-source disk encryption software for Windows Vista/XP/2000 and Linux
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 � 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."

http://www.truecrypt.org/downloads.php