Many Greek and Latin texts and editions are currently available on the internet in the form of digital facsimiles, as page images of printed books or manuscripts. Such images are useful both in research and in teaching (they enable us to use and show old printed books, papyri, medieval and early modern manuscripts). Digital facsimiles can be addenda to, or even an interim solution for, digital "reprints" of rare titles and works.
A meta-bibliography of collections of digital facsimiles is compiled by the British Library: .
Important sources of digital facsimiles are Google Book Search, Internet Archive, and HathiTrust. Useful descriptions of both exist on German Wikisource: Internet Archive, the GBS. Ryan Baumann's Book Aligner offers scripts to make it easier to identify parallel resources on Google Books, Internet Archive, HathiTrust, etc.
An example of good practice in serving digital facsimiles freely accessible over the internet is the Digitale Bibliothek of the Bayerische Staatsbibliothek.
See also: Classical texts on Google Book Search
Specialized lists of titles accessible through these service and important for classicists, can (and should be) compiled and shared, e. g. as contributions to Wikisource (e.g., the list of volumes of Hermes. Zeitschrift für classische Philologie compiled on German Wikisource), as free-standing websites (e.g., Tarik Wareh's Public Domain Books for Classicists), or as contributions to social bookmark and publication sharing services such as BibSonomy (see an example here).
Kevin Kiernan, "Digital Facsimiles in Editing" in Electronic Textual Editing, Modern Language Association's Committee on Scholarly Editions and Text Encoding Initiative Consortium. (Preview available on the TEI site, accessed on June 5, 2010.)