See how close you live to a nuclear power plant on this interactive map featured in Smithsonian. It's created by ESRI, home to all kinds of other maps, like the Battle of the Big Boxes, Ferris Bueller's Day Off, the location of uninsured Americans, a Timeline of the UK's Tallest Buildings, and more. [more inside]
What would happen if the earth stopped spinning? ArcGIS was used to perform complex raster analysis and volumetric computations and generate maps that visualize these results.
Make a Map is a website that lets you create your own maps of the US and areas thereof using various demographics data. It's still in beta stage but it's got all of the US (at least everywhere I've thought to look) and so far has datasets for median household income, population change 2000-9, population density, median home value, unemployment rate, average household size and median age. It's fun to use and taught me a great deal about my home city. The sitemaker, ESRI, also has a pretty good free globe map software, ArcGIS Explorer, for which you download map layers and add-ins.