The following web sites are hosted by responsible numismatic organizations (not dealers). You will find them very useful in your quest for numismatic knowledge.
Numismatic Guaranty Corporation. Official coin grading service of the P.N.G.
Numismaster.com
American Numismatic Association.
Bass Family Foundation.
Famous collection of US coins and currency.
Federal Reserve Bank of San Francisco.
On-line exhibit of currency.
