Adherents.com. Interesting site providing the latest statistical estimates of the world's religions.
Association of Religion and Data Archives. Quality Data on Religion Contains detailed maps and data on demographics of religious adherence in the U.S.
Beliefet.com. Excellent site for finding information, texts, discussion groups, news and links to a wide variety of faiths.
Maps of World Religions. Reproductions of textbook maps from Thomson Wadsworth publishers.