Lightbox2 is a simple, unobtrusive script used to overlay images on the current page. Not only is it a snap to setup, but it works on most browsers. Here are a couple of example thumbnails at work: 

Lightbox itself is already pretty nifty as a stand alone javascript application for your page, but the good people over at webdev:blog have decided to take it one step further by making it into a WordPress compatible plugin. I have installed it on my blog and was surprised that it didn’t require any changes to be made to my previous posts in order for it to take effect. All you need to do is download it, unzip into your ‘plugins’ folder and activate the plugin in your WordPress plugins panel then make sure your photos are coded with a title and linked to the actual photo file. It’s as easy as that!