We love any decent web visualization that explains interesting trends or developments. This map, developed for Forbes.com, nicely illustrates the movement of U.S. residents to and from different areas of the country during 2008.
The link is here.
The map takes some time to load initially.
By clicking on the county you want, the resulting black or red lines illustrate the relative flow of population, to or from that location.
Click, for instance, on Kootenai County to see where people there have moved from.
My only quibble is that it's not fully up-to-date. It would be even better to have a data map for the years 2007-2010.