In terms of "finished" projects, I wrote a program that generated new, empty, histories for Crusader Kings II. It could create new cultures, religions, and titles, and did a simple simulation of them spreading over the map.

I also wrote a server-side plugin for Assetto Corsa that monitored track limits, and could integrate into the program the league I was in used to find the pace of drivers for when we needed multiple servers.

In terms of "finished" meaning "I stopped working on it", I did write a compiler for a static concatinative language.