Ajax Photo Gallery
The AJAX version of AgileGallery is a free AJAX photo gallery that rips through the XML output from Picasa (a free download from google) and generates DHTML for the paging and thumbnails and displays the full sized photos along with any description entered in Picasa. Since this photo gallery uses AJAX technology, it eliminates the need for any page refresh as the user pages through the photos.

Minishowcase
Minishowcase is a small and simple php/javascript online photo gallery, powered by AJAX/JSON that lets you put easily your images in an online gallery, without having to configure databases or changing and customising code (though you may do it if you feel so) allowing to have an up-and-running gallery in a few minutes.

JonDesign’s SmoothGallery
Unlike other systems out there, JonDesign’s SmoothGallery is designed from the ground up to be standard compliant: You can feed it from any document, using custom css selectors. And even better, this solutions is very lightweight: The javascript file is only 16kb.