From September 18th, 2016 to November 7th, 2017, 100% user give 5-star rating. for Salesforce Instance Flag chrome extension.
Does exactly what it says on the tin.
It's great that a glance you can know exactly which instance your working in. Love the new update that allows you to control the color for each instance.
After accidentally deleting a bunch of stuff in our production org (because I had a sandbox in another tab, and got them confused), I found this fantastic extension and and I'm sure it's probably saved my butt a whole bunch of times since then.