The Washington Post has an excellent county-by-county interactive map that visualizes three decades of U.S. Census Bureau data. The page opens with a national view of the population in 2010 by race/ethnicity, but also allows several other views by decade.
Here’s the national view, with colors representing which group has a plurality in each county, and shades indicating the concentration:
More recent data shows other information collected last year by the bureau. This view, for example, is a national look at the percentage of residents who are married with kids in the home (notice Utah and South Texas — counties with the nation’s highest birth rates — have a greater proportion of these residents):
And, finally, if you zoom in, the map displays data at the block group level. Here’s D.C., which as I’ve noted is quite segregated: