The Wealth of Knowledge wallet is a ruler, a metric converter and a money holder all in one.
For anyone who does not have any recollection whatsoever about what they learned in elementary math class, this wallet can be your cheat sheet. It will help you do your temperature conversions when you find yourself in different areas of North America. It will calculate the volume of the wardrobe you are looking to buy and place in the obscure corner in your bedroom. It will even make pesky fractions easier to handle by displaying a chart of what they are equal to in inches.
The Wealth of Knowledge wallet is made from Tyvek, which is water-resistant and was designed by Ryan Plumb for the 'That Should Be a Mighty Wallet' contest.