Charles F. F. Karney,This page just provides a basic interface. Enter latitudes, longitudes, and azimuths as degrees and distances as meters using spaces or commas as separators. (Angles may be entered as decimal degrees or as degrees, minutes, and seconds, e.g. -20.51125, 20°30′40.5″S, S20d30'40.5", or -20:30:40.5.) The results are accurate to about 15 nanometers (or 0.1 m2 per vertex for areas). A slicker page where the geodesics are incorporated into Google Maps is given here. Basic online tools which provide similar capabilities are GeodSolve and Planimeter; these call a C++ backend. This page uses version of the geodesic code.
Algorithms for geodesics,
J. Geodesy 87(1), 43–55 (Jan. 2013);
DOI: 10.1007/s00190-012-0578-z (pdf); addenda: geod-addenda.html.