TrueCrypt is a software application used for on-the-fly encryption. It can create a virtual encrypted disk in a file (container), which can be mounted as if it were a real disk. TrueCrypt also supports device-hosted volumes, which can be created on either an individual partition or an entire disk. As of version 5.0, it can encrypt the Windows boot partition or entire boot drive. Version 6.0 even offers the possibility to create and run an encrypted hidden operating system whose existence is impossible to prove. It is distributed under the TrueCrypt Collective License.
Download: TrueCrypt 6.0a
Changes: Version history