I remember seeing this tip on either Survivorman or Man v. Wild. Basically if you hold your hand up (parallel) to the horizon, with your index finger lining up horizontally with the sun, each finger that can fit below your index finger represents approximately 15 minutes of daylight. For example, if your index finger is level with the sun and your pinky finger is level with the horizon, then you have an hour or 60 minutes left before the sun goes down.

This is very helpful if you are facing an emergency situation in the wilderness and need to calculate the amount of time you have left to prepare your shelter, start a fire, etc.