This site contains the names and addresses of historical and genealogical organizations in the state of Pennsylvania. It includes libraries, historical societies, and genealogy groups - anything which has to do with Pennsylvania genealogy.
http://www.pagenweb.org/organizations.html

This is a fairly comprehensive directory of genealogical and historical societies on the web.
http://www.familyhistory.com/societyhall/main.asp
You can search by society name or keyword, city, state/province, and/or ZIP code.

Historical Society of Pennsylvania
http://www.hsp.org/