For free? How strict are your security standards? Some of the best free-ware stuff is true-crypt, but requires you to have the TrueCrypt binary for decryption.
There is something called AxCrypt, which I have not used but is Windows only.
The next best thing I could think of would be IronKey. This is more of an encrypt-able USB thumb-drive though.
https://www.ironkey.com/personal-solutions