APE is the infamous Application Enhancer, now only available via the
Internet Archive. It allowed you to use the framework to do lots of fun things.
Apple never liked APE. And by "never liked," I mean, "hated it so much that they tried to destroy it at every opportunity." So every time Apple released a new OS X version, they made doubly sure to screw up the APE framework. It actually caused kernel panics and weird screens of death when you updated the OS and APE was installed.
Naturally, APE fought back and updated their software ASAP to make it work under whatever the new Mac OS happened to be. They actually got it working every time until 10.7. Something in OS 10.7 screwed up APE in a basically unfixable way, so Unsanity was forced to close up shop.
If you want the APE framework now, you can certainly use it under 10.5 or search the Internet Archive for the 10.4 and under compatible versions. You will need to toggle the date at the top of the screen (the archive overlay at the top of the page) several times to get a working page and download link. However, ClearDock and the APE framework is more than worth it on our old PPC's.