FreeOTFE
FreeOTFE is a free "on-the-fly" disk encryption program for MS Windows 2000/Windows XP. It creates "virtual disks" - anything written to which is automatically encrypted before being stored on the computers hard drive or USB drive. Numerous encryption (including AES, Blowfish, Twofish, Serpent, etc) and hash algorithms (SHA-512, Tiger, Whirlpool, MD5, etc) are supported, with a modular architecture that allows 3rd parties to implement additional algorithms if required.
This software is compatable with Linux encrypted volumes (e.g. LUKS), allowing data encrypted under Linux to be read (and written) freely.
FreeOTFE also allows "hidden volumes" to be created, giving plausible deniability and deniable encryption, and can be used to encrypt entire partitions or devices.