There many modal boxes libraries for Script.acolo.us, but there are two with very good results and which you have seen in many sites for sure. Lightbox and LightWindow are libreries with more than 1 year around, ..or 2, or 3....
The installation is as simple as adding the js and css files:
<link rel="stylesheet" href="/css/lightwindow.css" type="text/css" media="screen" />
<link rel="stylesheet" href="/css/lightbox.css" type="text/css" media="screen" />
And then just enclose the images or the media with style attributes!!!.
LightBox is a modalbox oriented to pics and images and lightwindow is a much more powerfull because it is compliant with any kind of media: Movies, Flash, PDF's, YouTube, Quicktime movies.......
They are totally recomended for Media Galleries!!. Using the CSS file enables to change the whole render of the "frame" where the media is enclosed when being showed.
Now it is time to check the sites: Lightbox, LightWindow and get to work!!. By the way nice hot weather at the south of Spain right now and the sound of the waves as the melody of this night!!
Soon modalbox for jquery, mootools...