Some one may have already mentioned it, but,
TrueCrypt. It's a Very Nice Program that lets you do all sorts of things by way of data encryption for security. And By encryption, I mean Like " O Hai Mistur prezidentz, here are those secret Documentz u wanted. Herp Derp." Encryption. and then, it takes these encrypted files and makes a virtual hard drive for you to store them on. and the encryption? well, it's almost im-fucking-possible to break, even if you knew what you were doing, as each time, it uses a randomly generated key based on random system data, like how and where you move your mouse after receiving the prompt to do so. so, yeah. Super secure and shit.
Also, after mounting the virtual drive, and filling it with, you know, whatever, you can dismount. and, under certain settings, it won't show up unless you enter the right password to access it. hence, you can't even prove that it's THERE, never mind access whatever you're trying to protect.