Back when Resexcellence.com was the ultimate Mac site I spent all kinds of time modding every aspect of my Mac. They had all kinds of how-tos and mods to make your Mac a little different. Sadly the site has been down for quite some time. However if your interested in modding your Mac, you may want to check out Loginox 1.0. Loginox is a small little app that allows for drag and drop customization of your login screen. Simply drag and drop a  jpg onto the open app window to instantly change your login screen background. Free and under 10MB makes it a nice addition to your arsenal of UI customization.

Download: Loginox 1.0