ACSEARCH.INFO Ancient Coin Search Engine. Extremely helpful. Created by Simon Wieland, Lars Rutten and Markus Beyeler.
CLASSICAL NUMISMATIC GROUP Invaluable "Research" section. Should be a model for other numismatic dealers.
CONSTANTINE THE GREAT COINS Great page created by Victor Clark. All you need to know about bronze coinage of one of the most influential Roman Emperors.
FORVM ANCIENT COINS Tools for identifying coins, articles and excellent discussion board.
NVMMVS BIBLE II Probably the biggest database of Roman coins minted from 313 to 476.
WILDWINDS Catalogue of Roman coins created by the late Dave Surber (now administrated by Dane Kurth). RIC numbers, inscription search and many more.