ColorBox: Highly-Customizable jQuery Lightbox
ColorBox is a jQuery Lightbox plugin which is lightweight (9kb) & supports photos, photo groups, slideshow, ajax, inline, and iframed content.
The lightbox has a very nicely designed interface which can be improved or customized with CSS.
ColorBox is unobtrusive, every option is mentioned in the JS & requires no changes to the existing HTML.
For faster displays & slideshows, it preloads background images and, optionally, can preload upcoming images in a photo group.
The plugin comes with a bunch of settings, including transition types-speeds & opacity, for maximum customization.