TrueCrypt is the leading open-source software to encrypt drive volumes. It supports Windows, MacOS X, and Linux. Version 5.1 has been released just 3.5 months ago in march. Now the Truecrypt team has released version 6.0 with the following features:
Parallelized encryption/decryption on multi-core/multi-cpu systems – thus demanding less time for the en- and decryption operations
Windows Vista, XP, 2003, 2008: run encrypted operating systems from hidden volumes (!)
Windows Vista, 2008: encrypt whole drives (incl. extended/logical partitions)
MacOS X: Create hidden volumes
Anyway: Permanent decryption has been removed from TrueCrypt’s bootloader, in order to support hidden operating system volumes. Permanent decryption now has to be done using the boot disk.