The secrets are organized into different categories according to certain applications and sections of Mac OS X Leopard. The first one, Top secrets, is a compilation of the best.
Some of the settings include the ability to change the background of the login screen, the appearance of scroll bars, and the screenshot output format. You can also show hidden files in Finder and more.
Be careful, though, because even the developers of Secrets warn that some changes may damage the operating system.