Beginners and experts alike will find plenty to digest at http://www.rootsweb.ancestry.com.
Visit Cyndi Howell's comprehensive site at http://www.CyndisList.com.
We find the LDS Church's http://www.familysearch.org to be invaluable. Check it often, as new records and indexes are loaded online on a frequent basis!
If you access the IGI or Ancestral File a lot, you will find the "classic" FamilySearch.org site more convenient than the new site.
Watch the USGenWeb project keep growing at http://www.usgenweb.org.
View your ancestor's land patent from the General Land Office at http://www.glorecords.blm.gov.