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 a few constants which you can preset in your own plugins, themes or even in your wp-config file. Definitions for the theme, opacity and/or maximum height are available:
SIMPLECOLORBOX_THEME Can choose from 1, 2, 3, 4 or 5
SIMPLECOLORBOX_OPACITY Any value from 0 to 1
SIMPLECOLORBOX_WIDTH Any value from 0 to 100
SIMPLECOLORBOX_HEIGHT Any value from 0 to 100
SIMPLECOLORBOX_SLIDESHOW Setting this to null will remove the slideshow