Recently I discovered a site that looks to be useful for everyone interested in ancient Greece. The Greek section of the Internet Ancient History Sourcebook from Fordham University contains a large collection of complete text from ancient authors and also contemporary literature for scientific research.
You can find Homer, Herodotus, Thucydides, Xenophon, Aristotle, Plutarch, Pausanias, and many other authors. The sources are categorized nicely by multiple criteria, such as the epoch (Archaic, Persian Wars, Peleponnesian War...) or the subject (Theatre, Philosophy...). Additionally there are similar collections for the Hellenistic world, Mesepotamia, Egypt, Persia, or Rome available with the link above. There are also other sourcebooks available like the Internet Medieval Sourcebook.