Association of Religion Data Archives (ARDA)

The Association of Religion Data Archives (ARDA) is a useful resource for people interested in the study of religion and demographics:

The ARDA allows you to interactively explore the highest quality data on American and international religion using online features for generating national profiles, maps, church membership overviews, denominational heritage trees, tables, charts, and other summary reports. Over 350 data files are available for online preview and most can be downloaded for additional research.

It has a wealth of data and features, but along with the Quickstats section, one of the more useful is the ability to compare statistics for different nations. For example, here is a comparison of Saudi Arabia and Australia.