Medieval PDAs. The Mechatronics Guy writes -

This page is mostly a copy of my Thingiverse entry here, but with extra observations: http://www.thingiverse.com/thing:25772 Before paper was commonplace, the method for keeping your thoughts in order was the wax tablet. They’re surprisingly easy to write on, and are capable of capturing almost the same level of detail as pen and paper. If you make a mistake you can use your finger or the rounded end of your stylus and smooth the wax back to blank again.