Rust: While I'd dabbled in C++ before, it was mostly just a pain, while I found Rust to be a nicer way to work on lower level projects. Ported a game side project I've been working on from Python + Kivy to Rust + Piston, and came out with a nice performance boost.
Mypy + SQLAlchemy: So while I have moved some projects from Python to Rust, others are still Python based and so I took the time to actually learn two of the libraries I've used at a surface level.
SQL: I mean, technically I've been using it for 10 years, but took the time to actually expand my knowledge from basic CRUD + table definitions. The one problem: Basically none of the nice stuff I learned works on the MySQL 5.7 we use in work :(
Docker: Another one of those things I'd been using but not really understanding, this is the most recent one where in the last month I've been working on a dev image to simplify running a few of our services in work, plus also redid my personal VPS to a dockerised setup for my various side projects so now it's way more reproducible than four years of ssh+vim methods of installation/configuration.