RedBear writes "A new update to the best open source transparent encryption software has been released. TrueCrypt is (the only?) open source encryption software capable of creating and mounting encrypted virtual disk images that can then be worked with transparently like any other storage drive, with data encrypted and decrypted in real-time. These virtual disks can be created as files, or entire partitions or physical drives can be encrypted and mounted transparently. Sadly there is still no Linux GUI or Mac OS X port in sight. If you are one of the thronging hordes who have been patiently awaiting ubiquitous multi-platform encryption, please consider donating time or money to the cause, and add your voice to the forum." From the site:"Among the new features [are] full compatibility with 32-bit and 64-bit Windows Vista, support for devices and file systems that use a sector size other than 512 bytes (such as new hard drives, USB flash drives, DVD-RAM, MP3 players, etc.), auto-dismount when a host device (e.g., a USB flash drive) is inadvertently removed, and many more." Read on for more features of TrueCrypt and cached versions of all the links above.
Also including features like plausible deniability, steganographically hidden volumes, unidentifiable partition headers, traveler mode, and your choice of the strongest available encryption algorithms up to and including multi-algorithm cascades. TrueCrypt is practically the Holy Grail for advocates of free ubiquitous encryption. Now, if only it were platform independent.
To reduce load on their servers here are some Coralized versions of all the links:
TrueCrypt home page
Future development goals
Forum thread about Mac OS X version