FreeOTFE

FreeOTFE is a free, open source, "on-the-fly" transparent disk encryption program for PCs and PDAs

With this software, you can create one or more "virtual disks" on your PC/PDA.  These disks operate exactly like a normal disk, with the exception that anything written to one of them is transparently, and securely, encrypted before being stored on your computer's hard drive.

http://www.freeotfe.org/