Follow these links:
http://www.u-s-history.com/pages/eras.html

http://www.historycentral.com/

http://www.historyplace.com/

http://www.mrnussbaum.com/circumcode.htm

http://havefunwithhistory.com/HistorySubjects/index.html

http://www.pbs.org/weta/thewest/resources/archives/index.htm

http://www.americaslibrary.gov/cgi-bin/page.cgi/jb

http://www.ushistory.org/us/index.asp

http://www.eyewitnesstohistory.com/

http://www.digitalhistory.uh.edu/database/hyper_titles.cfm