Church Records
UNITED STATES: GENERAL & REGIONAL - all denominations:
Overview of Church Records Research from the Wisconsin Historical Society
Religion and the Founding of the American Republic from the Library of Congress has a good historical overview of religion in early America.
Church Records in the United States - a guide from FamilySearch Wiki - this is the main directory that will lead to church records by denomination or state. Some pages are very detailed, some are waiting for experts to contribute more information.
United Kingdom & Ireland
Anglican Parish Registers in England - a guide from the FamilySearch Wiki
Guide to using nonconformist records - from the British National Archives
Guide to England's Church Records - a guide from the FamilySearch Wiki
Guide to Ireland's Church Records - a guide from the FamilySearch Wiki
Guide to Scotland's Church Records - a guide from the FamilySearch Wiki
CANADA
FamilySearch Wiki has an excellent overview of Ontario Church Records
Regional Archives:
Ontario: Guide to Ontario Church Records - a guide from FamilySearch