Microsoft Solitaire was developed by a summer intern
I don't know what it is, but I love implementing Solitaire to learn new languages and frameworks. It's always a fun exercise, and the engine gets better with every iteration. My latest attempt is <a href="https://solitaire.gg" rel="nofollow">https://solitaire.gg</a> - it's a Scala/Scala.js WebGL/websocket Phaser web/native app with hundreds of games.