U.S. Government Sites:
 

U.S. Department of the Treasury, Bureau of Printing and Engraving
(Where paper money is made)
U.S. Department of the Treasury, U.S. Mint
(Where coins are made)

U.S. Department of the Treasury, U.S. Secret Service - Know your money page
(How to identify counterfeit coins, currency and other government notes and checks)
Useful Links:
 

CoinSuperstore.com - contents copyright 2003-2012 inclusive