Since the Church of Jesus Christ of Latter-Day Saints requires its members to research their ancestors, they have amassed a huge genealogy database.
Anyone who has been researching genealogy online for a while, will undoubtedly come across the resources of the Church of Jesus Christ of the Latter-Day Saints (also known as the Mormons).
It's actually part of Church doctrine to study one's family history and genealogy. I'm not a member of the LDS Church, so I can only speak in general terms about the reasons for this. If you are interested in the scriptural basis for this, please visit the LDS homepage.
The basic idea seems to be two-fold. LDS followers believe that even the dead need to be baptised in order to gain access into heaven, so it is the responsibility of the living to acknowledge past ancestors and to have the proper ordinances (rituals) performed for them.
Also, there are church rituals to bind family members together through this life into the next, called 'sealing'. Commonly performed as part of a wedding ceremony, to seal husband and wife together. Ancestors and distant relatives are sealed, in order to unite an entire family (spiritually speaking).
These practices have lead to the creation of one of the largest genealogy database systems in the world. Most of their data is American, but as more and more people contribute to it and delve farther and farther back, these records can be very helpful.
Here are some websites related to the LDS genealogy resources:
Family Search - The official genealogy website of the LDS Church, with a search engine and other helpful research articles.
LDS Living - A site that offers a whole range of LDS-related genealogy products, such as books, DVDs and software.
Mormon Genealogy - Part of the About.com network, the Latter-Day Saints Guide has a series of articles and links, on the subject of genealogy and the LDS Church.
Cyndi's List of LDS Resources - Cyndi has a huge list of LDS genealogy websites, with many links to Family History Centers across the country.
Personally, I find the whole idea of genealogy as a religious responsibility quite interesting. Only while researching this article have I discovered these reasons for why LDS Church members research their family trees.