In 2011 and 2012 in between jobs I wrote a few bits of software, of which
- one was a SQLite3-based, all-SQL reimplementation of a subset of UName*It (an object-oriented database from the 90s meant for storing NIS/DNS/etc. data)
- another was an RCU-like lock-less, very fast user-space data structure written in C, born of frustration with read-write locks in Solaris
Of those the latter ended up being useful to me about 5 years later, and I still use it in production, though I originally wrote it for myself.