http://caltopo.com is the most powerful, free online topo mapping software I know of. It essentially provides a google maps - like interface for navigating an array of different maps, including USGS. The print function is amazing.
CalTopo is awesome... that's who we get USGS maps from for gaiagps.com.
I can say with some pride that the reason CalTopo has nationwide coverage is because we made a deal with them a long time ago to get that done. Matt originally set up CalTopo for his search and rescue efforts in California, hence the name!