The tool has gotten so much more functionality; the layout of this page doesn't really suit it anymore. Use instead the following link for the tool:
http://geo.javawa.nl/coordcalc/index_en.html

Calculating with coordinates

The tool (mobile version) can be used to perform miscellaneous calculations with coordinates, like those that are needed sometimes in geocaching.

The calculations are based on GeographicLib, resulting in a very high accuracy; even on long distances.
Calculations are performed along a great circle, the shortest distance over the earth's surface. This results in curved lines on the map, which can be clearly seen on long distances.

The lines and circles can sometimes have a strange appearance; this is the case when the borders of the visible map are exceeded (the earth is a slightly flattened sphere; sometimes the shortest path runs via the poles or the "rear"). You can get a correct display of the lines and circles on a 3D projection of the earth by clicking the button "Show 3D". This option is available only if your browser and video card support WebGL.

The tool supports coordinates in several different notations.

The following calculations are available:

Distance and center point between two points
Distance from a point to a line
Projecting a point
Intersection of the lines from two points with a defined bearing
Intersection of the lines between four points
Intersection of line with defined bearing and line between two points
Center of gravity, area and perimeter of a triangle
Orthocenter, circumscribed and inscribed circle of a triangle
Center of gravity, area and perimeter of a quadrilateral
Intersection points of two circles
Intersection point of three circles
Intersection points of a line and a circle
Circle through three points
Triangulation
Triangulation: Snellius-Pothenot problem
Area, perimeter and center of gravity of a polygon
Distance to horizon

Screenshot 2

There is a mobile version too: