TrueCrypt is a discontinued software application used for on-the-fly encryption (OTFE). It is distributed without cost and the source code and documentation is now available through this link.
It can create a virtual encrypted disk within a file or encrypt a partition or (under MS Windows except Windows 2000) the entire storage device (pre-boot authentication). -Wikipedia
Also, there is now a successor available at codeplex, which is based on the TrueCrypt sourcecode: VeraCrypt.