Bob Shapiro pointed me to this source of data. From the official announcement:

Social Explorer, in association with the Association of Religious Data Archives, releases maps and reports at the county level that provide counts of adherents and congregations of most denominations in the United States for 1980, 1990 and 2000, including Catholics, many Protestant denominations, both evangelical and mainline, Mormons, Muslims and Jews, etc. Based on the Religious Congregations and Membership Study this is the most complete census available on religious congregations and their members. . . . What to know where the Baha’i or the Church of Jesus Christ of Latter Day Saints (Mormons) are concentrated by state or county Social Explorer can tell you. There are well over 100 denominations reported for each decade.

We provide access to hundreds of interactive data maps and reports of the United States. Using them you may:

. Visually analyze and understand the demography of the U.S.
. Explore your own neighborhood
. Learn about the people living around you or anywhere in the United States

Social Explorer allows anyone using the WEB to create thematic maps (data maps) using several hundred variables from the 2000 Census down to the census tract, plus a selection from earlier censuses to 1940. Soon data back to 1910, where available will be added for Census Tracts, and data back to 1790 for counties. These maps are interactive, and one can identify the streets in the area. There is a “find” tool to allow you to find specific locations, including addresses. You can pan and zoom, look at specific areas, change the variables you are mapping and the like.

Using Social Explorer one also can create reports for any area or areas you chose from a wide array of variables. These reports also give you the context of your selected area including the United States, the state or states, and the county or counties your for your area. The report is then downloadable to EXCEL or it can be printed along with a map that identifies the selected area. In 2007, Social Explorer users made well over 2 million maps.

Social Explorer also makes it possible to create or view slideshows of the maps that you have created. This is especially useful to view change over time, or to zoom in to a given area from a larger area. Registered users will be able to store and reuse their own slide shows, and export them as animated GIFs to their own software.

This is pretty cool. The maps themselves don’t seem so good–they have a hard-to-read color scheme, a hard-to-interpret scale, and are filled with distracting details such as blue colors for all the little rivers and lakes in the areas. For example, here’s the map of the proportion of mainline Protestants in my area:

But it’s great to be able to get the data, and I really like the idea of being able to quickly download what you want.