You can also use the Boundary function in Google Earth and use (I think) the add "polygon" feature. You basically draw the boundaries and it can tell you the area.
THIS works, and you can do the same thing on the web with Google. Right click where you want to start, select "Measure Distance". Click the points you want until you get back to your original spot. It will tell you the acreage. Probably not good for professional use, but good if you wanna ball park it.