Online area calculator tool for Google maps.  In the example below, the area of Puerto Rico is calculated to be 8,939 km^2.  The official area is closer to 9,100 km^2.  I created the map below quickly (in about 1-minute), but I could probably obtain a more accurate estimate of area if I took a bit more time.  This tool could be valuable for estimating the area of farm fields, catchment areas for estimating surface runoff, etc.

http://www.mapdevelopers.com/area_finder.php