As of version 6.5, PGPdisk is not longer included with the freeware versions. If you want PGPdisk 6.5 or later, you are required to buy one of the commercial versions. However, v6.0.2i is available here in both mac and pc versions. http://www.pgpi.org/products/pgpdisk/
TrueCrypt can create a virtual encrypted disk within a file and mount it as a real disk. It can also encrypt an entire hard disk partition or storage device. Supports plausible deniability. http://www.truecrypt.org
Control Break provides encryption and security tools which are both secure, and easy to administer - designed primarily for the large enterprise, they also offer a range of consumer verisons. http://www.safeboot.com
Delivers full hard disk encryption: automatic, on-the-fly data protection for your entire hard drive. Easy centralized administration for efficient corporate use. http://www.pcguardian.com
Polymorphic on-the-fly harddisk encryption developed by CyProtect AG. Easy to use and 100% secure. It is possible to encrypt your data on external (USB or Firewire) and internal harddisks. http://www.cyprotect.com