Inspired by the Gmaps Pedometer, I made a map that computes the area enclosed by a given path: the Google Areometer.
Actually there already was at least one other map that does this, but it only does a planar approximation. My version uses completely accurate spherical geometry, including drawing great circle arcs instead of straight lines.
There are a couple other features of interest to Google Maps authors:
it remembers the most recent position/zoom/map-type in a cookie;
and it correctly handles lines & regions that cross the International Dateline.