I have for years been wondering which ecommerce carts are the most popular, and therefore have the most staying power and will likely continue to be a viable option for clients to build and maintain a business on. Being popular also means that they likely have a thriving third-party developer ecology. (Third-party developers are programmers that aren't employed by the cart company, but make add-ons, modules, themes for the cart.) Popularity also means that most glitches will have been sorted out, and that there will be a method of implementing most business process needs. Oh, and it means that the community of users will be a valuable resource for figuring out how to solve your particular implementation problems.
I've never seen an authoritative list of the most popular shopping carts, until now.
This is a f-f-f-fantastic tool because it shows cart popularity by business size. The needs of a top 10000 website are going to be a lot different from the needs of a top 1 mil website.
I am still on a quest for an integrated commerce solution for small businesses, but Builtwith may have provided me with a shortlist of contenders.
Here's another good shopping cart comparison article: CPCStrategy Ecommerce Platform Comparison