The most basic feature of a pop up is to display an image and there are many that do this well while maintaining a small file size. But some of you may need to group images into sets, display inline elements, display iframes, fetch content via Ajax, display flash or even display video. So you will need to find a pop up script that supports the features you are looking for. Shadowbox and Multibox support all of these features for those of you that need them all.
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