Many readers may already be familiar with Digital Library Numis (DLN). The project "is aimed to establish a major portal and a digital repository for freely available numismatic publications on coins, medals and related subjects, which mostly can be downloaded online."
The resources are not restricted to ancient coins, but to numismatics in general. The site features a search engine and subdivides subject matter into themes: e.g. "Roman Coins: Catalogues" or "Roman Coins: Iconography," or "Greek Coins: Aegean Islands, Crete, Cyprus," etc.
The bulk of the accessible material is 19th century monographs that are now out of copyright. A number of Ernst Babelon's important works are accessible as well as Cohen's catalogue which will be useful for anyone working with Roman coins from old collections or reports. But a few newer resources crop up as well. Essentially the aim is to aggregate material that is already available online and so for the newer research there is a prominence of articles from Revue Numismatique since it has already been made freely accessible online.
A contact link, prominently displayed on the homepage, allows users to report problems or suggest the addition of material.
Thanks to Thijs Verspagen for the suggestion to cover DLN here.