Lightbox JS [1] is a simple, unobtrusive script used to overlay images on the current page. It\’s a snap to setup and works on all modern browsers. Once the scripts is loaded, you simply use link tags to get the overlay effect.
Update: Have also a look at zenphoto.org that some combined with Lightbox [2,3,4].
[1] http://www.huddletogether.com/projects/lightbox/
[2] http://zenphoto.org
[3] http://technorati.com/search/zenphoto
[4] http://taggphoto.com/zp/blurp/
