TrueCrypt comes to mind
http://www.truecrypt.org/
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, Blowfish (448-bit key), CAST5, Serpent, Triple DES, and Twofish.
Mode of operation: LRW (CBC supported as legacy).
If you need to access this data away from your machine, and do not want or can not install truecrypt on the machine going to be used to access the data - then use Traveller Mode
http://www.truecrypt.org/docs/?s=traveller-mode