AutimmArmy.com
From a programming perspective this project was very similar to my Intellij license server but it came with a few new features such as a section in the site where you could monitor in-game global chat live. (powered by websockets however the in-game chat was actually powered by HTTP body streaming)
This site was capable a slew of things such as enumerating peoples Facebook and Google accounts simply by supplying their user id. It was also capable of spoofing in-game messages and allowed you to join in-game clans without approval by anyone in said clans.
At one point this site was the pinnacle of "Modern Combat 5 hacks", probably because of the clan hijacking feature mentioned above.
This project was ended when the company in question handed me a seize and desist order after which I took down the site and they fixed all the vulnerabilities except the user data leaks.