Harry Kau has developed these great commute maps which show how people are commuting to or from any zip code. The map above shows where people commute to from my own zip code, 55409. These data are a bit old, being based on 2000 Census, but the concept is still excellent.
I could spend hours clicking around the metro (and country) and looking at travel patterns. I’m relatively unfamiliar with the Census Transportation Planning Package, but it seems like a gold mine.