religious groups:
http://www.patheos.com/Library.html
http://www.mnsu.edu/emuseum/cultural/religion/
http://www.adherents.com/Religions_By_Adherents.html
http://www.religioustolerance.org/var_rel.htm
http://www.pbs.org/edens/thailand/buddhism.htm (for Buddhism only)
http://en.wikipedia.org/wiki/Major_religious_groups