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.