[New] Slightly ThickerBox 1.1 is a mod of the Thickbox script, that integrates previous and next links (excellent for image galleries) and the ability to create groups of galleries.
GreyBox - An iframe approach to creating popups on the screen, fo images, html or even a different url - about 20k of overhead.
GreyBox Redux - unobtrusive (no need to embed JS into your site) and even lighter weight than GreyBox.
Lightbox v2.0 - the original for images only. Uses prototype and sciptaculous libriaries so it's a bit code-heavy, but very nice for image use.
Lightbox with moo.fx - souped up with moo.fx transitions etc, still prototype based and only for images, however.
Lightbox Gone Wild - a lightbox variation as a modal window, so allows html, forms etc as well as images. Still uses prototype so again, a bit code-heavy.
Multi-faceted LIghtbox - prototyped based again, but offers a variety of options for different types of data that can be loaded in the lightbox.
Leightbox - very similar to Lightbox Gone Wild (prototype based, allows images, html etc) except that Leightbox keeps the lightbox content in the html, but hidden from view, so that it can be indexed by search engines.
xilinus - easy to use, prototype-based, easily customizable and allows for a variety of lightbox content options.
Lightbox Plus - original lightbox plus additional features allowing for expandable images and automatic image resizing based on window size.
Suckerfish Hover Lighbox - lightbox effect while keeping accessibility. Not classic lightbox-ish but a nice presentation, nonetheless.
[New] Litebox - a code-reduced version of Lightbox, uses the moo.fx library and prototype-lite. A good choice if page weight is a concern.
New LITBox - a lightbox written in to a class, so it is possible to open multiple instances of the lightbox and control each separately. Based on Thickbox.
Slimbox - is a 7kb visual clone of the popular Lightbox JS, written using the ultra compact mootools framework (separate install that will add 16kb to the total weight). It was designed to be small, efficient, more convenient and 100% compatible with the original Lightbox v2.
[New] YUI PhotoViewer - a lightbox gallery script built for the Yahoo User Interface Library.
Lots of options, including the ability to load photos from an XML file.