Population statistics and maps (population, density and changes) for every county in the US. Most populated counties are Los Angeles County, California, Cook County, Ilinois and Harris County, Texas. County with largest population density is New York County with more than 47000 p./sq.mi (if density for this county would be same as for Alaska, then population of NY county would be only 37 people). Population rate changes for period 2010-2014 is very variable value, that can be seen on third map below. Approximately 44% of counties have negative- and 56% have positive population rate. You can find county using search field or start by clicking on the first letter of the name or choose the state.
Or you can search county by state..
 County area, latitude longitude data, retrieved 02/23/2016
 County boundaries, CSV files on boundaries.us, retrived 03/18/2016