keynav
My fork may be found here.
Once again, not my work but merely a fork of something unmaintained that I liked.
keynav allows one to control the X cursor with the keyboard rather than the mouse. I wouldn't suggest totally replacing mouse use with it, but in the case of just needing to quickly move focus in a stubborn application or click a couple buttons that have no ready shortcuts, it's way nicer than having to take your hands off the keyboard.
I maintain this fork because the original author stopped responding to GitHub issues and emails; there were a decent number of forks implementing various bugfixes and improvements, but nobody to merge changes back into the main version and no coordination. My version, therefore, was created by merging most of the existing forks back into one codebase, plus some improvements of my own (mostly in the build process).