Do you use custom post types and taxonomies? Do you want to list any related content, not just posts?
Custom post types are one the best features in WordPress. Since WordPress 3.0 they are much easier to use. Almost every theme I build for a client features at least one custom post type and usually a custom taxonomy as well.
But there is a problem. There is no plugin that lists related posts that are from a custom post type. After looking through the sourcecode of a few plugins I decided to implement my own related content plugin.