- bash + GNU coreutils; seriously, take the time to be able to write helpful bash scripts which can run basically anywhere.
- git; use it even when you're not pushing to a remote. Add helpful aliases for everyday commands. Build a good mental model of commits and branches to help you through tough merges. ( my ~/.gitconfig: https://gist.github.com/jeaye/950300ff8120950814879a46b796b3... )
- Regex; combined with bash and your GNU tools, a lot can be done.
- Vim; modal editing and vi-like navigation can blow open your mind. Explore the existing plugins to accomplish everything you want to be able to do. It's all there. ( my ~/.vimrc: https://github.com/jeaye/vimrc )
- Functional programming; if you're new to this, start with Clojure, not one of the less-practical languages. This has made such a huge impact on the way I think about code, write code, and design code that it's possibly the biggest one here for me.