I'll second USAPhotomaps at www.jdmcox.com. Its the best free aerial/mapping tool I've found. It plays nice with my GPS too. I like to put gridlines on my map and the aerial photo too, both to the same scale. When I set my GPS to UTM coordinates, I can pretty much look at my GPS and then at the grid on my map/photo and know where on the map I am. If I'm scouting a new area, I'll check it out online first, click and add some waypoints for interesting looking spots, then download those waypoints to my GPS. I can walk straight to them without ever having been there before.