TrueCrypt is an excellent open source disk encryption system for Windows, Mac, and Linux systems. Users can encrypt entire filesystems, which are then on-the-fly encrypted/decrypted as needed without user intervention beyond initially entering their passphrase. A clever hidden volume feature allows you to hide a second layer of particularly sensitive content with plausible deniability about whether it even exists. Then if you are forced to give up your passphrase, you give them the first-level secret. That only allows them access to the innocuous material you have there, without proving that a second level key even exists.
