There are many tweaks and adjustments that you can make to OSX that are hidden by default but can be revealed by using a program such as Secrets.
Secrets is a free system preference pane for Leopard only, that allows you to change and customize hidden settings for your system and most applications. It allows you to make all kinds of tweaks and modifications, many of which are useless really, including using a circular progress indicator for Safari instead of a bar and changing the arrows on your scroll bar. Note that Secrets should be used with extreme caution because it is in beta stage and any wrongly made adjustments could affect the stability of your Mac.
The number of options available in Secrets is really overwhelming and half of the modifications and tweaks are hard to even understand. Changing them is a matter of trial and error but since it could affect the stability of your Mac, this is quite a worrying process. In the right hands however, it's probably a far more powerful tool than something like Onyx for customizing your Mac.
Secrets is packed with tweaks, changes and optimizations that will unveil hidden features within Leopard. Just use it with care.