This is a list of very useful sites.. The links will point to various attribution resources and ancient coins dealers.

Wildwinds               zillions of coins listed, pictures, descriptions, prices...
Coin Archives       All the coins from the important auctions of the the past few years
Dirty Old Coins    Cleaning resources and  ID database
RomanCoin.info   The Roman Coin Attribution Toolkit
denarios.org           An excellent Spanish collection of denarii

Specialized sites:
Probus                  Specialized site on the coinage of the Emperor Probus
Severus Alexander   Specialized site on the coinage of the Emperor Severus Alexander

Discussion forums / mailing lists:
FORVM's Discussion Boards        The best place to talk coins, in a modern full featured format, 1200 members !
Moneta-l                                                yahoo discussion group - lots of collectors and dealers

Dealers:
FORVM                                                2000 ancient coins for sale
CNG                                                       major coin dealer
HJBerk                                                   another major dealer
Flan                                                         coins for sale and useful links
Beastcoins                                            very  interesting collections and coins for sale