USGS has a map viewer I just came across:

http://viewer.nationalmap.gov/viewer/

It allows you to add or subtract the layers you want -- imagery or topo; place names, hydrography etc. Looks cool and allows you to download the maps you create.

In addition ArcGIS is putting together a World Topo map as one of their base layers for their software. Sequoia Kings is currently editing a detailed addition for it. There's a major glitch right now in that you don't see trails until about 1:12,500 -- which is really too close for planning purposes. This is now available using ArcGIS Explorer or, I assume, their online version of Explorer.

The map itself is here to just view, with no reference:

http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer?f=jsapi

And the metadata for the map is here:

www.arcgisonline.com/home/item.html?id=f2498e3d0ff642bfb4b155828351ef0e

g.

None of the views expressed here in any way represent those of the unidentified agency that I work for or, often, reality. It's just me, fired up by coffee and powerful prose.