Now I've finished the Big Monster Project of Doom that I've been on the last two years, I can talk more about some of the code that I wrote for it. I can also say what it was: it was a web application for activists to canvass the public for a certain recent national referendum (I'll let you guess which one).
One of the major modules I wrote was Entity Operations module. What began as a means to avoid repeating the same code each time I needed a new entity type soon became the workhorse for the whole application UI.