Aegean Prehistoric Archaeology
Perhaps the most important (and influential) web site for the study of Prehistoric Aegean archaeology. Conveniently divided into lessons for self-study. https://sites.dartmouth.edu/aegean-prehistory/

Nestor
Bibliography of Aegean Prehistory and Related Areas: Searchable database.
http://classics.uc.edu/nestor/

The Beazley Archive
Wealth of information on the Classical world, especially Greek art and architecture. Includes bibliographies, images, etc.
http://www.beazley.ox.ac.uk/

The Parthenon Frieze
Extensive images and a virtual tour of the Parthenon frieze, one of the most celebrated (and contested) works of Greek art.
http://repository.parthenonfrieze.gr/frieze/

ABZU
Web resources for the study of the Ancient Near East.
http://www.etana.org/abzubib

Metis
Catalog of Greek sites with images and (in some cases) Quicktime VR tours.
http://www.stoa.org/metis/

Roman Art and Architecture
Great source for images of Roman art and architecture.
http://formaurbis.stanford.edu/

Stanford Digital Forma Urbis Romae Project
The Severan Marble Plan of Rome, a crucial resource for the topography of Rome.
http://formaurbis.stanford.edu/

Styppax
An academic resource for the study of Cypriote sculpture: An on-line resource for the study of Cypriote sculpture and archaeology. Includes an extensive bibliography, as well as links of interest. Site maintained by Prof. Counts (UWM, Art History).
people.uwm.edu/dbc/