I worked for Ai Squared on an app named Sitecues, which was a SaaS product that companies would add to their website to improve their accessibility for low vision users. We ran into a vast number of edge cases when trying to make our JavaScript library compatible with all of our customers' websites, many of whom had awful coding that we needed to handle gracefully (hacky CSS, old versions of Prototype JS that override Object.prototype, and far, far worse). One day I stumbled upon the iGrill website and found out that it exposed practically every problem that we had seen scattered across various other sites. It was so convenient that their website was so poorly coded that I could stress test our app on their site and if it worked, well, "ship it!"
Looks like their site has had a few updates since then, but those were good times.