The uWatch is an open source RPN and Algebraic scientific calculator watch that you can build yourself. Jones writes: [via]
It only had an 18 key keypad, but I figured that was OK as my old CFX-400 only had 16 + 4 side keys. All the extra functions were performed with a MENU key and two rows of 4 soft function buttons that mapped to the 8 keys directly below the LCD.
uWatch : World’s First DIY Scientific Calculator Watch - [Link]