We are just putting the last finishing touches on our newest site, Sloan Plumbing Parts
Pretty basic site, except for how we used grouped products to show part breakdowns.
I added an extra field to the backend for the associated products list for the part number on the specific breakdown diagram, which i described here
On the frontend, we display that information, as well as a few other custom attributes associated with each product.
The list also has a link that someone can click on, which uses lightbox.js to display product information, and allows them to add the item to the cart via ajax, which then shows the cart in the same lightbox window.
You can see an example of that on the following page Sloan Royal Flushometer