Not a tool but I wrap a length of that double sided Velcro around my trousers/boots at mid shin height (one on each leg obviously). Seems to do a good job of keeping out water when doing river crossings.
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Life long rider. Currently the CCM 450, a GSA and a Guzzi Le Mans I've had since 1981. Sold the TTR September 2016.
A nice cheap (free!) aid to allow wheel removal whilst out laning
Because we occasionally come across trees or branches down and blocking our progress in the lanes, most of us carry a fold up pruning saw in our tool kit. This makes the job of finding and making a bike support a cinch
By positioning the branch further back, the rear wheel can be lifted off the ground.
The Halfords mini digital tyre pressure gauge is prob mine, can easily up the pressures accurately to 18+ if I am going to go on a lot of knarly jaggedy rock trails. CLICK
Also even though not a tool, the mini gorilla camera tri-pod is a good one too for putting the camera in awkward spots etc. CLICK