Years ago I started using Knox for securing files, folders and drives. It's an easy and very secure way to keep your private files private. This week, long-time developers Marko Karppinen & Co. handed it over to Agile Web Solutions. As Agile is behind the popular 1Password, I think it was a great choice.
In fact, the two utilities work well together: Knox protects your files while 1Password protects your passwords. Knox employs the US Government's Advanced Encryption Standard (AES) with 256-bit or 128-bit key length. What that means is that your stuff is safe from the cranky teenager down the block as well as hostile nations. I've used it to secure USB flash drives, as they're so easy to misplace.
Version 2 is available here. It's a useful application that's in good hands at Agile. A 30-day free trial is available, and a single license will run you US$34.95.