Outdoor Gadgets Your Favourite/most Useful Outdoor Recreation Gadget/accessory?

Your favourite/most useful outdoor recreation gadget/accessory? - outdoor gadgets

What are your favorite gadgets and the most commonly used / accessories for outdoor sports like camping, mountaineering, rock climbing?
I hear about all your favorite gadgets that come in handy for you and where you have him or how long you have been through.
If you have a link, where you bought to see for myself?
Thank you:)