Roman Provincial Coinage Online covers the periods AD 96-192, 222-244 (selected provinces) and 249-254. It includes over 30,000 images of coins.
Sylloge Nummorum Graecorum is a British Academy project and publishes illustrated catalogues of Greek coins from both public and private UK collections.
Online Coins of the Roman Empire (OCRE) covers the whole period of the empire, from Augustus to Zeno. There are some helpful features including an interactive map view of finds.
Roman Republican Coins in the British Museum are also available to view online - about 12,000 coins are included.
Art and Archaeology Browser Part of the Perseus Digital library covers coins from three collections.