American Numismatic Society

Coin Grading Services:
PCGS - Professional Coin Grading Service
ICG - Independent Coin Grading Company
NGC - Numismatic Guaranty Corporation of America

Online Auctions / Coin Dealers:
eBay
Yahoo
Amazon.com
TeleTrade

Heritage Auction Galleries

Coin auction site with links to ancient coin database listing auction results

Ancient coin auctions, there are links to big name companies like CNG & H Berk and smaller ones like Pegasi & Nilus

Auctions of US, world and ancient coins and books

MA-SHOPS has a large selection of ancient to modern, world coins. The shops (dealers) have coins for sale and auctions

Several companies auctions of US, world and ancient coins

Other Coin Clubs:
Bellaire Coin Club
Greater Houston Coin Club
 

Newsletters:
Coin Connoisseur
Coin Today

Coin Shows:
Coin Shows

Museums:
The National Numismatic Collection at the Smithsonian Institution
The Museum of Biblical Coins