WP Gallery Custom Links Plugin
When working on client sites I ran into situations where a series of thumbnail images on a page needed to link to other pages on the site. Using WordPress core image gallery functionality seemed like a simple way to handle this, but WordPress only allows the thumbnails to link to the attachment post or the raw file itself.
I created a plugin to allow a third option: a custom link. This plugin has been added to the wordpress.org plugin directory as a free plugin.