This was posted @Lifehacker today. I would also like to mention that you can by sheets of tub/shower board for $10 and have your local Menards/Lowes/Home Depot cut them in the store for you, one sheet can make approx. 32 handheld boards.
Amplify’d from lifehacker.com
Make a Transparent Dry Erase Board for less than $6
See this Amp at http://amplify.com/u/a15d1u
