Vimium. Vim-like shortcuts for the browser. I can't name any other tool or extension that gives me anywhere near the productivity boost of Vimium. I spend probably half of my work day plus a few hours a day in my spare time in the browser, and it makes navigating the browser feel like butter. When I'm tired and don't want to be glued to my desk, I can relax and surf with one hand which just feels incredible. I quickly got so used to it that I instinctively try to use Vimium shortcuts when I'm on other computes and feel withdrawal symptoms if I realize that it isn't installed.