OnyX

Now here's a fun little Application.

OnyX not only lets you access hidden settings like changing the look and feel of the dock and using screensavers as your desktop wallpaper, it also has many functions which let you make your computer work better. Of course I only got it for the fun little settings (my computer works fine), but these can be very useful for general cleaning up, which you can use if you feel the need to. Anyway, here are some screenshots of my computer OnyXed:

In this image, I've changed the look of the dock to 2D with transparency effect, made the 'shell' screensaver my desktop background, and hidden the 'spotlight' icon.
I was going to do one that showed what you could do with the login screen, but I realized that you couldn't take screenshots unless you're logged in. So that's all you get.