TrueCrypt is a free open source encryption program developed by the TrueCrypt Foundation and first released in February 2004. TrueCrypt can open existing encrypted storage areas, drive partitions, and even entire drives. However, as of 2014, the program is now defunct and the final version, 7.2, has purposely been crippled to prevent it from making new volumes.
Although the final version of the program passed an independent security audit in 2015 showing no obvious problems, multiple potential security flaws have been discovered since then, and the program's official Web site now recommends that Windows users cease using it and encourages Microsoft's BitLocker. However, BitLocker is closed source, has several documented flaws, and requires faith in Microsoft a company who has been contacted numerous times by government agencies to include a backdoor for them. The most obvious open source choice is VeraCrypt, which was forked from TrueCrypt after it was abandoned and has seen regular updates since. VeraCrypt can open TrueCrypt files, but it uses a new unique format to address potential flaws in TrueCrypt's design.
I used to use TrueCrypt for keeping all my banking, government, and password files encrypted, but, after it was abandoned, I switched to VeraCrypt.