Ancestry.com: A great place for researching. Well worth the money in my opinion.
Cindi's List: A huge site of genealogy links
Family Search Genealogy Service: Run by the Church of
Jesus Christ of Latter-Day Saints, a very good research site.
Genforum: Bulletin Boards for thousands of different surnames. I have found tons of information here.
Family History.com: Another bulletin board site that is smaller in scope than genforum but is connected to Ancestry.com. That means that searches at ancestry.com will pop up any messages within the search criteria.
Rootsweb: Historic, census and other information for various counties throughout the USA.