Appalachian Mountain Club's Equipped: Nearly Every USGS Topo Map Ever Made. For Free.
Jared Sperli stashed this in intel
The U.S. Geological Survey (USGS) has been producing detailed topographic maps for more than 125 years. Today they are nearly all digitized and free to download through the USGS Map Store, an incredible treasure trove for both map junkies and casual hikers alike.
Locating your desired map is straightforward with the USGS Map Locator and Downloader, which allows you to zoom in or search for your area of interest. It then gets mildly confusing (though easy once you figure it out) to download the actual map.
One important thing to note is that, in general, the most recent topo maps listed are markedly different from their predecessors. Part of the new US Topo Series, these maps have been created as PDFs with geospatial extensions (GeoPDF), which gives you the ability to turn on and off different layers (contour lines, place names, water features, etc.) for viewing, depending on what information you are interested in. Unfortunately, however, trails are not currently included as one of these layers—a significant drawback for hiking.
Lastly, and one of the single-most useful online tools I've discovered in recent years, is the ability to overlay every USGS topo map on top of Google Earth, another free (and extremely powerful) tool to add to your trip planning quiver.