Ancestry.com: Provides some online services for free but others require a paid subscription. Useful information can be had without a subscription, though.
FamilySearch: Created and maintained by The Church of Jesus Christ of Latter-day Saints, this site offers a vast array of genealogical information and research assistance. One of the most useful sites on the Web.
The USGenWeb Project: Portal to free genealogy sites for counties and states across the United States.