One of our partners in our American Futures project, along with Marketplace radio, is the Esri mapping/geographic-info company of Redlands, California. Here are two interactive maps Esri has recently produced that I think are potential time-sinks of the instructive rather than of the "you'll hate yourself when you spend half an hour this way" variety.
First, a "swipe map" that lets you compare recent rates of county-by-county population growth with the sources of that growth—or decline. You can see the full-screen version of the map here, which also explains its legend. In short, the darker the shade of green on the left side of the map, the faster the population growth. And on the right, a tan color means that migration has been the main source of change—people moving in—while blue means the "natural increase" of births and deaths. A pale color on either side means no growth/no change.
Click the words "Hide Intro" when you first see the map, to get a view in which you can pan around and zoom in or out. Don't click on either "Data" or "Legend" — or, if you do, click back on "Map" to get the real display. Again, darker green is faster growth, and tan is people moving in. If you forget, just look at North Dakota.
Next, we have a really extraordinary overlay of some 175,000 historic topographical maps, whose power becomes evident if you click on a place you're familiar with. You can read background from the USGS here, and from Esri here. This latter link describes some of the technical feats necessary to produce this display. It also includes a series of maps showing, as an example, Phoenix's dramatic expansion through the past century.
To try the historical maps, go to http://historicalmaps.arcgis.com/usgs/, move around to find a place you care about, click on that site, and follow the instructions to see a range of historical maps. For instance, here is the way our current neighborhood in Washington looked in 1890, when today's Tenleytown was apparently called "Tennallytown" and when, surprisingly, what are now the main drags — today's MacArthur, Nebraska, Loughboro, Wisconsin, Reservoir, Foxhall, etc—had already been laid out. Also a surprise: that nearly 125 years ago there was already a reservoir overlooking the Potomac, which gave the then-unbuilt-upon Reservoir Road its name.
And here is how my home town looked around the time I was starting kindergarten. I am not sure* exactly what the red shading indicates, but our house was at the very bottom of the red area. Most of the other area shown was orange groves.
These are places of interest to me; you will find ones of interest to you. Congrats and thanks to Esri and their partners at USGS and the Census for making these maps available.
* Update What about that red-tinted area? Reader Kit Case points out something I should have noticed myself. If you go into the Esri historical-map browser and choose old maps to inspect, you'll see, over on the left side of the screen, little thumbnails of each map you've chosen. By each thumbnail is an option to download the original map itself, as a PDF. When I download the map shown above and open its PDF, I see a full legend—including, in this case, info that red shading means areas where "only landmark buildings are shown," like schools and libraries, rather than each individual house. Which is why my family's house doesn't show up, but one right across the street, in a non-red area, does. Now I know.