neat gadget up right now that lets you see migration patterns within the United States, the one above is for Merced. You can get more details, like precise numbers and the actual names of the counties involved (in case your knowledge of U.S. geography is less than encyclopedic) by going to the website. Merced stands out for how much people are really just coming from and going to other places in California (although it would be interesting to add on international immigration too). The colors are less useful than they could be...shading seems to be based on the ratio of out-migrants to in-migrants, making Los Angeles County show up as a source of in migration and Santa Clara County as a destination for out migration, but the in migrants from Santa Clara County outnumber the in migrants from LA by about a thousand. Similarly, Fresno County and Stanislaus County are the same color, but about thirteen hundred more people decided to head north towards Modesto than to head south towards Fresno, which is probably significant. Anyways, it's a cool thing to play around with. I'd love to hear people's ideas for worthwhile comparisons to make.
(Redding & Chico, my favorite Merced controls so far, have seemingly wider distribution but less concentrated migration coming from or going anywhere, richer migrants, and dramatically less migration overall.)
Holy shit you can look at how migration patterns have changed since 2005 too...hot damn!