Most successful project at work: NodeJS utility for storing loads of measurements from an application running on various other systems and providing fast ways of getting at that data. No DB, just CSV files broken into time periods. Also has a search function written in C that can very quickly find all user sessions matching the criteria. It's not perfect, but it does the job pretty well and I can tweak the storage engine as much as needed for our use case since its all custom written.

Outside of work: Incomplete right now but I soldered some wires onto an old sound card and managed to get an Arduino to configure it and play some notes on its FM synthesis chip. Still quite a newbie to electronics so this was quite an achievement for me personally.