THE genealogy web site of the LDS Church. Includes the ability to search the IGI, the Ancestral File, the Family History Library Catalog, and web site submissions. Also has user mailing lists, online ordering of FHL research materials and allows you to preserve your own research online.
'Ancestral File' and 'The International Genealogical Index' are, probably, the most widely used genealogical reference sources. Unfortunately, they are also two of the most common sources of erroneous information.