The Map Gallery of Religion in the United States housed at http://www.valpo.edu/geomet/geo/courses/geo200/religion.html is the stuff of dreams for map nerds. This is made for hours of fun as the map reader immerses him or herself in the data and, more importantly, the "why" of the distribution of various traditions represented in the various maps. Enjoy!
Here is the page's description:
Map Gallery of Religion in the United States
The U.S. Census Bureau, due to issues related to the separation of church and state, does not ask questions related to faith or religion on the decennial census. Accordingly, there are few sources of comprehensive data on church membership and religious affiliation for the United States. Perhaps the leading organization to address this gap is the Glenmary Research Center, which publishes Religious Congregations and Membership in the United States, 2000. The following series of county-level choropleth maps, which reveals the distribution of the larger and more regionally concentrated church bodies, draws on this resource. The maps are in GIF format.