TrueCrypt : Open-Source Disk Encryption Software for Windows and Linux
TrueCrypt is an excellent open source disk encryption system. Users can encrypt entire filesystems, which are then on-the-fly encrypted/decrypted as needed without user intervention beyond entering their passphrase intially. A clever hidden volume feature allows you to hide a 2nd layer of particularly sensitive content with plausible deniability about whether it exists. Then if you are forced to give up your passphrase, you give them the first-level secret. Even with that, attackers cannot prove that a second level key even exists.