A follow-up from my post from a few days ago – the NY Times has used the new 5 year 2005-2009 ACS data to build census tract level maps for the entire country. They’re great for viewing distributions and for identifying basic data for specific tracts, but you can’t download or capture anything:
http://projects.nytimes.com/census/2010/explorer?ref=us
(note – a census tract is a relatively permanent statistical entity created by the Census Bureau designed to have an ideal population between 1500 and 8000 people, with an ideal size of 4000)