Colorbox is an excellent jQuery plugin by Jack Moore. There are other Colorbox plugins for WordPress, but Simple Colorbox is, well, the simplest. Gone are complicated administration panels and mess to confuse your clients with. The Simple Colorbox is as cut down and simple as it can be. Simple install, activate, and the plugin will add beautiful layovers via the Colorbox script automagically to all linked images.
For the more hard core among you, the plugin class includes filters to allow you to modify the settings. These can be modified by creating a custom plugin or via custom code in our theme. You can see how these work inside the plugin code.