I wish Google Maps would add the display of 24K USGS topo maps. That would provide people planning a trip with a LOT more information about the kinds of roads involved, though good paper maps should mark limited-use roads.

I currently use Geobuddy (the "brother" of Expert GPS with added geocache-tracking features) to view topo maps on-line. Here are their respective web sites if you're interested (I have no connection to the makers):

http://www.geobuddy.com - $59.95
http://www.expertgps.com - $59.95