Dual Spring Gate Carabiner
This has incredible utility for such a simple device. When I travel I hang my hats on my briefcase with this (or water bottles). I use it to attach my keys to things so I don’t forget (or lose) them. I have one that keeps my Beats headphones attached to my standup desk… I have them in several sizes and colors. ROI is off the chart for a sub-$10 purchase.