This is an online edition of Michael Crawford's "Roman Republican Coinage" (RRC). It indexes coins in the British Museum collection, as well as many other collections.
OCRE Online Coins of the Roman Empire
OCRE is a collaborative project at the University of New York, the American Numismatic Society and the Institute for the Study of the Ancient World. From the website, "The project records every published type of Roman Imperial Coinage from Augustus in 31 BC, until the death of Zeno in AD 491. This is an easy to use digital corpus, with downloadable catalog entries, incorporating over 43,000 types of coins". The databases entries are integrated with the Digital Atlas of the Ancient World and Pelagios.
RPC I From the death of Caesar to the death of Vitellius (44 BC–AD 69) Perkins/Bostock CJ969 .B87 1992; also Lilly
RPC II From Vespasian to Domitian (AD 69–96) Perkins/Bostock CJ969 .B87 1992; also Lilly
RPC III From Nerva to Hadrian (AD 96–138) Perkins/Bostock CJ969 .B87 1992; also Lilly
RPC IV.1 From Antoninus Pius to Commodus (AD 138–192): Cyrenaica to Bithynia-Pontus
RPC IV.2 From Antoninus Pius to Commodus (AD 138–192): Asia
RPC IV.3 From Antoninus Pius to Commodus (AD 138–192): Lycia-Pamphylia to Arabia
RPC IV.4 From Antoninus Pius to Commodus (AD 138–192): Egypt
RPC V.1 From Pertinax to Macrinus (AD 193–218): Europe In progress
RPC V.2 From Pertinax to Macrinus (AD 193–218): Pontus-Bithynia and Asia (In progress)
RPC V.3 From Pertinax to Macrinus (AD 193–218): Lycia-Pamphylia to Egypt (In progress)
RPC VI From Elagabalus to Maximinus Thrax (AD 218–238): Asia Minor and Egypt
RPC VII.1 From Gordian I to Gordian III (AD 238–244): Asia Perkins/Bostock CJ969 .B87 1992; also Lilly
RPC VII.2 From Gordian I to Gordian III (AD 238–244): all provinces except Asia
RPC VIII Philip (AD 244–249)
RPC IX From Trajan Decius to Uranius Antoninus (AD 249–254) (on order for DUL)
RPC X From Valerian to Diocletian (AD 253–297) (In progress)
SNG Sylloge Nummorum Graecorum - British Isles
This is a project of the British Academy and indexes and documents coins in collections in the United Kingdom. They define "Greek" coins as "coins produced by all ancient civilisations of the Mediterranean and neighbouring regions except Rome, though it does include the Roman Provincial series often known as 'Greek Imperials'".
MANTIS: American Numistmatics Society
The ANS database indexes of 600,000 objects. The first page organizes different kinds of coins by departments, Greek, Roman, Byzantine, etc.
This is a catalog of over 8,000 coins.
This is a website which catalogs the coin collection in the Universität Bonn, which include over 1500 Greek and 3500 Roman coins. In German.
This is a database of coins in the Staatliche Museen zu Berlin - Stiftung Preussischer Kulturbesitz.