Product Filter Mod
The first major project that I headed, this customization was part of a wider site design for a client who hoped to mimic several larger chain's search abilities. Originally built with the idea of narrowing down things like snowboards and snowboard accessories, it soon became much more useful for things such as clothing sizes and price filtering.
The available options are all automatically generated based off of various values that are created in the vanilla version of X-Cart, so new owners of the module have very little to input if they don't want to, although there is a variety of administrative options to change filter names, groupings, orders, etc.
It is one of my most popular modules, and it has gone through an enormous amount of QA and revisions.