Visage

System Preferences pane that allows you to easily and safely customize the appearance of Mac OS X. Run screen savers as your desktop background, customize your login screen, personalize system alerts, and change the startup message.

Visage keeps track of Apple's default settings for every customization, so you can easily switch back to the original settings at any time. If you like to personalize your Mac, Visage is an essential tool.
Version 2.2.2 fixed situation where Visage's icon might be used for the Finder's dock icon even after deselecting the option, addressed the issue that could cause Visage to crash when importing images.