E6B flight computer
Really cool way to do vector arithmetic. For noting airspeed, wind speed and wind direction, and to calculate the proper heading to go from point a to point b. I guess you could use it to add any two vectors though, if you used a little imagination.
http://www.ebay.com/itm/APR-Student-Pilot-E6-B-Flight-Computer-with-Training-NP-8-Plotter-E6-B8P/150772186888?rt=nc&_trksid=p2047675.m1851&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D163%26meid%3D2566397678814929595%26pid%3D100005%26prg%3D1088%26rk%3D1%26rkt%3D4%26sd%3D290950474453%26