Apple Computer is known for their secrecy, this app lets you join in on the fun.
Espionage will take any text, and change it into a secret code. You can then send the code to a friend in an email (or the built in iChat integration) and they can decode the message on their end.
There are quite a few different codebooks included, but perhaps the coolest part is that you can build your own code book and distribute it only to those you want to be able to decode the messages.
Good uses for this app:

Be sure that your boss can't read the emails that you are sending from your computer.
If you are on a connection that is not secure, but need to send your credit card info thru iChat just encode and send
You can write love poems about one girl, and not be afraid to let a different girl use your computer for a paper.
Post a message to the readers of your blog and don't worry if Google picks it up.
If you have a good memory, I suppose you and your friend can be the only two people in the world to speak a coded language.

Have fun. Don't break the law...