For everyone who is using google earth, here is a link for a topo overlay. I do most of my cyber scouting in google earth between this overlay and the aerials and then will look at other maps if I need to. For the aerial I usually change the date to a time when the imaging was done in the winter so I can see the ground cover and terrain a little bit better. 2002 is what I use in my area, it has better resolution than some of the older maps.
http://www.gelib.com/ng-topo.htm
http://www.gelib.com/ng-topo.htm