[ionforce]

That's just like early days video game anti-piracy measure.

What is the third word on the second paragraph of page 42 of the Dungeon Master's manual? Etc.

[nknighthb]

Military codebooks are used in this way for authenticating over unsecured links. Letters or numbers laid out in a grid-like format, and you make the far side read off a certain cell.

http://en.wikipedia.org/wiki/DRYAD

http://en.wikipedia.org/wiki/BATCO#Other_functions

[jcfiala]

The Pragmatic Programmer website uses this system to allow you to prove that you bought a hard-copy of a book, so they can offer you a discounted ebook.