
Really fab fancy zoom image popup script, smooth quick transition from and back to a gallery of images, i prefer this to an unmodified lightbox script
ajax script example here
ajax script demo here
ajax script download here

Really slick sliding login / register form, ultra smooth transition from the top of your web page, making this a really pleasing popup, that will impress your visitors rather than annoy them
ajax script example here
ajax script demo here
ajax script download here

Lightbox is the de-facto standard image popup and image gallery script, lots of variants have been produced
ajax script example here
ajax script demo here
ajax script download here

Extended and expanded version of lightbox
ajax script example here
ajax script demo here
ajax script download here

Suckerfish hover lightbox redux is a really cool alternative way to display your images, it’s a image hover rollover gallery with lightbox style image popup
ajax script example here
ajax script demo here
ajax script download here