Here is another good resource courtesy of BLM.
http://www.geocommunicator.gov/blmMap/MapLSIS.jsp You can control layer density and go all the way down to topographic contours without the bother of map edges or white spaces. As you move closer in scale, it goes from meters to feet. very nice stuff though may not always be real current.
this link is very useful because you can look at historical topographic maps, back to the 1940-50 range and pick up old trails that are not shown on current maps. I don't care for the most up to date topo maps so generally use Forest Service link in this thread or the BLM link above.
http://geonames.usgs.gov/apex/f?p=262:1:878711327294:::::Let's go Riding!