This online tool, at http://gisapps.co.union.nc.us/community-mapper/, is designed for parcel measurement in Union County, North Carolina. I just needed a quick way to find the area of a map we'd made in Lima, Peru. I'll try Google Earth next, though.
It ended up being 207 acres, or 0.3 square miles. Not so great, given that it was from 2 miles up!