Gaadi horseshoe tube
For various reasons, changing the tube on a bicycle wheel can be difficult. If the axle isn't quick-release, then a spanner must be carried to undo the wheel nuts. If an internal gear hub is used, cables or other mechanisms will need to be disconnected (ditto roller or drum brakes). If there's an electric motor, you'll need to undo the cables as well.
Enter the Gaadi bicycle tube. It's shaped like a horseshoe, with two ends. That means you don't need to remove the wheel at all - you can just roll the tyre off the rim, cut the old tube out, and replace it with the Gaadi tube.
It's not cheap, but it does save a lot of time and effort, and means you don't need to carry spanners with you everywhere. Perfect for an emergency - keep one at the bottom of your bag.