Taxonomy Picker isn’t writing but a WordPress plugin. The code is published under the GNU General Public Licence (GPL) and is available for download via the WordPress plugin repository. Details are available here. (If that link expires, check the respository which will maintain the definitive link.)
The plugin adds a widget which allows users to interactively build their own query – for instance selecting that they are interesting in Product = Mens Golf Shirts and Colour = Blue and Size = Large.
More WordPress plugins are in development.