Historia est magistra vitae …

Here are some web-resources on Ancient History and the Classical World (in no particular order)

General & History
Bibliotheca Classica Selecta: Online journals on Ancient History
Persee Scientific Journals: provides a platform for a distinguished set of periodicals, especially French publications.
ORBIS, The Stanford Geospatial Network Model of the Roman World: An interactive model to reconstruct the time cost and financial expense associated with a wide range of different types of travel in antiquity .
The Perseus Digital Library: Digital library project of Tufts University. Collections include primary and secondary sources and images of ancient objects, sites, and buildings.
The Oxford Roman Economy Project: Website of the AHRC founded project on the Roman economy, giving access to databases, working papers, and list of events and conferences.
Prosopographia Imperii Romani: A ‘who is who’ of the Roman Empire (in German).
Roman Army Bibliography: A detailed bibliography on the Roman army.
Lacus Curtius: various resources for classics and classical studies.
ETENA: A guide to open access data relevant to the study and public presentation of the Ancient Near East and the Ancient Mediterranean world.
Centre for the Study of Ancient Documents, Oxford University: important resource for quality digital images of inscriptions and collections of ancient documents, such as the Vindolanda writing tablets and the Oxyrhynchus papyri.
Ancient World Mapping Centre, UNC at Chapel Hill: offers a series of downloadable maps of the ancient world.
Pleiades: digital resource for historical geography of the Greek and Roman world.
CEIPAC: website of the Spanish Centre for Provincial Studies in Classical Antiquity, good source of articles by Spanish scholars in PDF format.
Online books: UPenn, list some of the major sources and indexes of free online texts
Art & Archaeology
FASTI Online: Contains useful summary notices of excavations on many areas of the Roman Empire.
Forma Urbis Romae, Stanford University: a site dedicated to exploring the Forma Urbis Romae (Severan Marble Plan of Rome)
Ostia: A rich resource on Ostia antica, giving description of monuments, plans, photos and more.
Roman Aqueducts: a great resource on Roman aqueducts across the empire.
Viae Romanae: A detailed bibliography on Roman roads.
Beazley Archive, University of Oxford: database of Greek and Roman decorated pottery.
Database of Ancient Art: Good collection of Roman art images.
Archnet, Arizona State University: with numerous links to resources, organizations and institutions.
Aphrodisias, New York University: all about the NYU excavations at Aphrodisias, Turkey.
PompeiiPictures: Good resource for images of Pompeii and its monuments.
Epigraphy
Introduction to Latin & Greek epigraphy: An absolute beginner’s guide.
Epigraphic Database Roma: Part of the EAGLE project, includes almost 29,000 inscriptions. The search form is rather complex and requires a bit of time to familiarize with it.
Epigraphische Datenbank Heidelberg (EDH): in addition to texts, the site offers referred relevant bibliographies as well as a huge database photographs of inscriptions.
Greek Epigraphy: a database of searchable Greek inscriptions funded by The Packard Humanities Institute.
Latin Epigraphy: the Clauss-Slaby epigraphic databank offers probably the best coverage for Latin inscriptions; the search function is simple to use.
Hispania Epigraphica: Roman Inscriptions from the Iberian Peninsula.
Numismatic
The British Museum: Website of the Dept. of Coins and Medals.
American Numismatic Society: Large database of Ancient, Medieval, and Modern coins and library catalogue.
Roman Provincial Coinage Online, Ashmolean Museum: dedicated to provincial coinage of the Roman Empire from 44 BC - AD 296/7, with an extensive catalogue.
Sylloge Nummorum Graecorum: A British Academy Research Project, publishing catalogues of Greek coins in public and private collections in the British Isles.
Digital Historia Numorum: a manual of Greek Numismatics.
Forum Ancient Coins: A commercial web site with searchable ancient coin catalogue, discussion board, and NumisWiki
Coin Archives: A searchable archive of ancient coins featured in major numismatic auctions.
Institutions