I use ArcGIS to do my acreage measurements. I grew up in Minnesota where it is very simple to find the acreage on flat and relatively square fields, I moved to Virginia and I was completely lost in my own internal acreage guesses. This site has been fixed that. It is very simple to use and accurate. When I first found it I tested it against known square footage measurements and GPS calculated measurements and it was always close enough that I kept using it.

http://www.arcgis.com/home/webmap/viewer.html?useExisting=1