Today I built my first real Drupal Module. It was way easier than I expected it to be. The Module doesn't do anything I hadn't already found a way to do without making a module, but the mod solution is certainly more elegant, and possibly more secure.
So, what does my mod do? To find out, navigate to one of the Books at this site and take a look at the Page Extras panel that appears to the left of the book content. That's my module!
Books to try: