Heard about this website on the radio yesterday morning...
A rule of thumb turns information you have into information you need.
The goal of this website is to gather every rule of thumb on earth into one gargantuan, easily searchable online reference database that will be accessible from anywhere in the world and continue to grow forever.
For example: Count the number of times a cricket chirps in 15 seconds, and add 37. No, you didn't just calculate your age in cricket years - you measured the temperature outdoors in degrees Fahrenheit. You just used a rule of thumb.