Reprinted from Free Geography Tools blog (posted there April 14, 2007).
In addition to the standard image overlays of Map, Satellite and Hybrid for Google Maps, the Active Trails website has an additional overlay button – Topo, for topographic maps. At lower zoom levels, the 1:100K scale USGS topo maps are visible, but if you zoom in close enough, the larger-scale 1:24K maps become visible. Comparing Map, Satellite and Topo overlays, there appears to be a good position match between the three. Active Trails also has placemarks for many kinds of outdoor trails across the country (hiking, biking, equestrian, etc.) entered by site users. You can also install a network link in Google Earth that will let you view the trails there, similar in nature to the new Google Earth layer from Trimble.
Addendum: Fixed the Active Trails links (sorry about that). In the comments, MH points out the ACME Mapper 2.0 site, which is a better choice. It displays black-and-white USGS aerial photos, and also lets you print out a copy of the on-screen map with the push of a button.