Family Search is a free service, operated by the Church of Jesus Christ of Latter Day Saints, that focuses on the more social aspects of genealogical research. In addition to mapping your own family tree, searching the site's record archives, and uploading old family photos, Family Search allows you to share and collaborate all of these features with other users. Who knows, you might just find a long lost 5th cousin.
The USGenWeb project is a free service for genealogical researchers that collects and disseminates historical information on both the state and county level. The group's volunteer admins manage the site's expansive collection of birth, death, and marriage records. In addition, the group also provides specialized information resources for researching your African American roots as well as for finding more about specific people in history. The site and its services are both completely free to use.
DistantCousin is one of the most versatile free genealogical resources on the Internet. It offers more than 10 million records—from newspaper obituaries and census records to school yearbooks and passenger lists—from more than 4,000 online sources, all for free. While it won't build your family tree for you, as most costly sites will, it does offer a huge amount of information if you're willing to sort through census data for it.
Excerpted from Climb your family tree with these online genealogy tools by Andrew Tarantola.