This is an online (PHP-)version of a little TCL/Tk script I slapped together during a stay in Antarctica to keep track of the line-of-sight distance to a balloon payload. For some reason I couldn't easily or quickly find something like this on the web.
So I wrote my own.
This program is about as simple as they come: you enter the longitude and latitude of two points on the planet (in degrees) and it tells you the shortest distance between them (the great circle distance) in kilometer.