Kruptos 2 uses powerful, 128-bit encryption to encrypt files and folders so that only you can read them. It's particularly useful for USB flash drives and portable storage devices, because you can encrypt the entire device, so you need not worry if the device is lost.
It does a lot more as well. You can, for example, create self-extracting, encrypted archives. And you can also use it to shred files that you delete so that you remove all traces of them from your hard disk. You can even disguise a file name when you encrypt a file, so that people won't even know the names of your files, much less get access to them.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.