TrueCrypt is a freeware software application used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file or encrypt a partition or (under Microsoft Windows except Windows 2000) the entire storage device (pre-boot authentication).
Read more about TrueCrypt: Operating Systems, Cryptographic Algorithms, Performance, Security Concerns, Operation Satyagraha, Licensing, Trademarks, Planned Features, TrueCrypt and The Trusted Platform Module, Incompatibility With FlexNet Publisher, Version History