I've developed a plugin for my current employer that allows us to add advanced filtering of product categories in the sidebar area. It is similar to the google shopping sidebar, or any advanced filter that you'd see on an e-commerce site. ex: Ebay, HomeDepot.com, Amazon... This plugin will allow you to build your own advanced product filters.
For each category, we can ping the products for attributes and custom fields, and then generate an appropriate sidebar based upon the product set. We currently have code for dual sliders(min/max range), checkboxes, and nested menus that activate on a mouse click. This allows customers to filter the product set by price, dimensions, brands, colors, sizes, special features, and other properties. The filter loads results via ajax.
On the admin side, the filters are fully customizable with a drag and drop interface. It also provides statistics for each category, helping to complete product attributes and custom fields that might be missing.
I was wondering if anyone else would be interested in these kind of features. Should I develop this into a distributable plugin? How many other people could benefit from this? How much would you be willing to pay for this plugin?