Perseus
by Tufts University
is limited to Ancient Greece, but is becoming exhaustive, it
offers the original and translations and is supported by dictionaries
and other tools. Useful, if you know exactly what you are
looking for. The responsibles should consider the option of displaying
complete texts and not only small portions in an infinite series of
pages.
Myriobiblos E-text Library of the Church of Greece. A wealth of sources
and studies in many languages. Don't forget to visit this place.
Early Christian Writings. New Testament, Apocrypha, Gnostics,
Church Fathers. The site is well organised, and contains also links
to studies of the first centuries.
Christian Classics
Ethereal Library at Wheaton College publishes the Ante- and
Post-Nicene Fathers. Limited to the (first) Christian centuries,
not very selective and, sometimes, even difficult to use, but a work of love
and discipline, always getting better.
Koine Greek Documents, a very useful and well made web site by John Barach featuring bilingual editions and a lot of help in learning Greek!
Cf. Resources on Speaking 
Places 
Perseus by Tufts University is limited to Ancient Greece, but is becoming exhaustive, it offers the original and translations and is supported by dictionaries and other tools. Useful, if you know exactly what you are looking for. The responsibles should consider the option of displaying complete texts and not only small portions in an infinite series of pages.
Suggest a resource!
