A good discussion forum about about old coins:
Numism-L Discussion Forum
Numism-L Archive
The American Numismatic Society Coin Collection:
The ANS Collection Catalog (Search Menu)
Other noncommercial sites:
Asian Coins and History (Tom Mallon)
Ancient and Medieval Indian Coins (Dr. Nupam Mahajan)
Danish East Indian Coins (Uno Barner Jensen)
Chinese Cast Coins (Robert Kokotailo)
Virtual Coin Album (Sam Kazmi)
Ancient Coins (Rune Rysstad)
Ancient Coins (FAQ)
Magna Graecia Coins (John van Wielink)
Celtic Coins (Philip de Jersey)
Römische Münzen
Julia Domna's Coins
Byzantine Coins (Serbian Unity Congress)
Crusader Coins
Irish Coins (John Stafford-Langan)
French Coins (Jim Mason)
Serbian Coins (Serbian Unity Congress)
Astronomical Events on Coins (Marshall Faintich)
Virtual Coin Collection (Dan Drew)
Some commercial sites:
Alex G. Malloy, Inc.
The NumisArt Galleries
Calgary Coin & Antique Gallery
Jean Elsen s.a. Numismatists (Belgium)
Guy Clark Ancient Coins and Antiquities
Kenneth W. Dorney, Classical Numismatist
Classical Numismatic Group, Inc.
Art Noot, Numismatist
Robert G. Lilly Ancient & Medieval Coins
Malter Galleries Inc.
A good Journal and the ANA (with its lending library):
The Celator
The American Numismatic Association
Other lists of numismatic Web Sites:
Penny Lane
Fiddlecats Ancient Coins