Metabunk's Flat Earth Route Simulator shows the shortest route between two points on each of a globe, a flat earth, and a standard map. The lengths of each route are calculated for the globe and the flat earth.
Click and move the ends of the lines (on the above map only). The globe can be rotated with the mouse.
Assumes that the distances along north/south lines of longitude (i.e. distances from the North Pole) are the same on Flat Earth as they are on the regular globe Earth.