If you'd like to hide a certain file on your PC or USB drive, Safe Calculator is designed to do just that.
It looks just like the windows calculator in normal mode, and only turns into a safe if you enter a PIN number (default is 123). From safe mode, you can choose the file you want to hide, and Safe Calculator will eat the file, keeping it encrypted and password protected until you choose to restore it. In normal mode, it works just like a normal calculator!
As Safe Calculator looks like a calculator, it's not that easy to use as a safe. However, there is a great help file included, and it soon becomes relatively simple. It can be stored an run from a USB stick, so is fully portable. It does the job of hiding a file very well - it's just a shame it only hides one file at a time, and you can't store folders.
Safe Calculator is a good place to keep a file you really want to keep hidden, although it's not too user friendly, and only stores one file at a time.