Found a programming language consisting entirely of whitespace (tabs, spaces, and line feeds). This is some of their example code:
Although this is of course a fully functional language, I liked the subtlety of this joke:
Whitespace is a particularly useful language for spies. Imagine you have a top secret program that you don't want anyone to see. What do you do? Simply print it out and delete the file, ready to type in at a later date. Nobody will know that your blank piece of paper is actually vital computer code!