I would like to introduce you to a very powerful and useful online statistical archive of U.S. Church Data, the ARDA at http://www.thearda.com/.
"The archive is a collection of surveys, polls, and other data submitted by the foremost scholars and research centers in the world." You have access not only to the analyzed data but to the poll and survey forms themselves. A feature on the site actually helps you to utilize those surveys to create new ones.
This site should be of especial importance to DMin candidates who are analyzing the state of their congregation and the larger state of religion in the U.S.
My favorite feature allows you to compare different congregations and see how members of those congregations respond to social questions. The data provides invaluable information about the socio/economic makeup of groups and how those factors line up with social questions.
For those who love history or genealogy, the site provides graphical trees for several denominations in the U.S. (we seem especially adept at schisms). In addition to Christianity, the site also provides data about other major world religions in the U.S.
I am including this site in my online resources in the Library section of the NYTS web page.