Tmux, vim, Linux like every other nerd in this thread. aider is my favorite recent piece of software. Some scripts I made do a lot of work for me, although they're very simple.

Also my 14" band saw, which I tuned most of the vibration out of. And some screwdrivers that belonged to my grandfather. And a beautiful set of blue point wrenches that belonged to someone else's grandfather. A cheap set of wrenches my mom got me for Christmas in 1999. And my old Coleman gas lantern. Those are my favorite tools that I use at every opportunity.