The Distance Between Two Points
Now we all know the shortest path between two points is a straight line. Unfortunately, unless you believe the Flat Earth Society, any journey between two points on Earth usually requires you to make a bit of an arc in your journey. Therefore, when you measure a distance on a two dimensional map, your calculations are likely to be wrong, unless you allow for the curvature of the Earth.
To top it all off the script is available for free under a Creative Commons license.