Favorite road cycling gear and/or gadgets

Road cycling requires a lot of gear. Much of the gear we use overlaps with what other people are already using, like helmets, lights, etc.

But every now and again, I learn about people's favorite gadgets/pieces of gear that make their kit unique and that improve their cycling lifestyle.

For example, mine is the Topeak Ratchet Rocket. This tool has eliminated the traditional multi-tool for me. It's so much easier to use and virtually the same in terms of size and weight. 

What is the piece of cycling kit that you swear by?