Download free digital topographic maps in a GeoPDF format by going to the USGS Store and clicking on "Map Locator". These are high-resolution scans (400 ppi to 508 ppi) of existing paper maps. Free digital historical topographic (all scales, all editions) maps are currently being added to the USGS Store's Map Locator.
Download the next generation USGS Topo digital topographic maps from the same site (go to the USGS Store and click on "Map Locator"). US Topo maps will be available for all 50 states by the end of 2012. These maps will be updated every 3 years.
Paper versions of all the above topographic maps (traditional and US Topo) can be ordered through the same Map Locator site at the USGS Store.
To browse popular thematic maps, go to the USGS Store and click on "Education Products"
Questions? Need help? Call 1-888-ASK-USGS (1-888-275-8747) or submit your question online.