DefProc

WordPress plugin to automatically add rel=”lightbox” attribute to link wrapped images, as placed into WordPress posts.
This plugin requires lightbox or slimbox (or any lightbox-compatible image handler) to also be installed on your site, either as part of the theme, or via a plugin. If either lightbox, or add-rel-lightbox is not activated at any time, your site will revert gracefully to the standard “open image as new page”.
The plugin currently only works with images placed into posts using the “add media” dialogues, and will only apply to images that are in your media library. However, it will also not interfere with any images that have already have attributes applied to them.
add-rel-lightbox also works with the [gallery] shortcode, adding the rel=”lightbox” attribute to links created by it. Note that you will have to use the <code>[gallery link="file"]</code> format, or select “Image File” when inserting the gallery for lighbox to work as expected. Galleries that link to the attachment page are unaffected by this plugin.
In addition to applying rel=”lightbox”, this plugin will retrieve the media library caption for any images, and correctly escape any html entities, so that the caption is displayed as expected in the lightbox.

add-rel-lightbox

Add rel="lightbox" to <a> wrapped image links in the content and to [gallery] image links, and include captions for lightbox/slimbox

Kevin Sylvestre

This plugin uses the jquery implementation of slimbox and makes use of [attr] style selectors by adding a section to the wordpress header.
For more information and examples of slimbox visit:
<a href="http://www.digitalia.be/software/slimbox2" rel="nofollow ugc">Slimbox</a>

Slimbox

Enables slimbox 2.03 on all image links including BMP, GIF, JPG, JPEG, and PNG links.

add-rel-lightbox vs Slimbox