Genealogists constantly pore over old records. Of course, the biggest provider of old records in the U.S. is the U.S. government. The government even maintains a listing of significant record collections available for genealogists. The list also mentions many state and local archives.
You can view the listing at: http://www.usa.gov/Citizen/Topics/History_Family/State_Genealogy.shtml.
While the listing is online, most of the records listed are not yet online. You will normally have to visit the repository listed in order to view the record(s) in person. Some have been microfilmed and some of these records are now appearing on the various online sites: Footnote.com, WorldVitalRecords.com, Ancestry.com, etc.