This started out as a word document describing the places to "hard code" into the Magento web-shop. Over the last year or so it evolved into a full blown extension (for Magento). Today I released the "core" of it as open source.
Let me know what you think, good or bad.
http://vehiclefits.com/download.php (scroll to bottom and agree to terms
It started out without any unit tests, and over the last few months I have been writing them and fixing stuff. I have yet to completely "separate" all the stuff Magento imposed on me, but I made the code as re-usable as possible. Just as a disclaimer the messier stuff is in Block/ Adminhtml/ and Helper/. It also started out using mysql_query() and has been recently ported to use Zend_Db (since Magento uses that already anyways.)