The Mark 8 Flight Computer was -- together with the Mark 3 Plotting Board -- the main navigation tool for US pilots and navigators during WW2 when it came to flight and navigation planning. It was used to compute simple groundspeed, distance and time calculations as well as getting True Airspeed (TAS) from Indicated Airspeed (IAS), depending on altitude and temperature.
The Mark-8 Flight Computer was used as a separate device the size of a check book. Navy pilots often used it attached to the plotting board (lower right corner).
Photo courtesy of The Historical Aviator (Link).
Analog flight computers are still in use today as every student pilot is learning how to use it in flight planning. Pilots often even carry it on every flight in order to be independent of electronic computers in case of system failure. Modern versions are often referred to as "The E6B", but many other versions exist, e.g. CRP-5.
Original manual for the Mark-8 Flight Computer. Download the PDF from here.
Photo and PDF courtesy of The Historical Aviator (Link).