Wikipedia
TrueCrypt
TrueCrypt is a discontinued source-available freeware utility 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 7/8 boot drive with GPT) the entire storage device ( pre-boot authentication).
On 28 May 2014, the TrueCrypt website announced that the project was no longer maintained and recommended users to find alternative solutions. Though development of TrueCrypt has ceased, an independent audit of TrueCrypt has concluded that no significant flaws are present.
Alternatives include two freeware projects based on the TrueCrypt code, VeraCrypt and CipherShed, as well as numerous commercial and open-source products.