Forbes has an interactive map where you can click on any US county and see where people from that county moved to and where people moving into the county moved from during 2008. Very cool. It would be even better to have more years of data available and be able to see exactly what happened in a place like Cook County in the 2000s with Chicago’s overall population loss.
One complaint: it is hard to distinguish the red lines (outward population movement) from the black lines (inward population movement) when looking at the same county. For example, if you look at the line between DuPage County, IL and Los Angeles County, CA, the line is red even though 252 people moved from LA County to DuPage County and 309 moved in the opposite direction.
The Brookings Institute has put together a website with interactive data maps of metropolitan America. The data comes from the 2009 American Community Survey (done by the Census) and one can look at all sorts of variables across cities or metropolitan areas.
The best feature, in my opinion, is the tab that lets you compare suburbs alone across metropolitan regions. Very quickly, you can find that the suburbs of Syracuse, New York have the highest percentage of non-Hispanic whites (93.1%), the suburbs of San Jose-Sunnyvale-Santa Clara, California, have the highest median household income ($96,478), and the suburbs of Modesto, California have the highest percentage of commuters traveling more than 90 minutes to work (7.2%).