THE GREEK TEXTS OF THE MASSIVE MIGNE EDITION OF THE GREEK FATHERS is available online, as noted by Andrew Criddle at Hypotyposes. This caught my eye because several of the texts being translated for the More Old Testament Pseudepigrapha Project are included in this corpus and thus are now available online in their original Greek. These include the following. Click on the links to download the pdf files.

The Story of Melchizedek is incorrectly attributed to Athanasias: Historia de Melchisedech. Another Melchizedek story is preserved in the Chronicon paschale.

The story of Zechariah and King Joash is summarized (from an ancient Hebrew writing reportedly found by a monastic superior of a community at Gerari) in Hermias Sozomenos, Historia ecclesiastica 9.17 (PG 67.1269b).

The story of The Rich Man and the Precious Stone is summarized in Georgius Monachus Hamartolos, Chronicon (not sure of the reference) and also in Georgius Cedrenus, Compendium historiarum 1.193-94 (PG 121.226-228).

UPDATE: Alex Panayotov e-mails:

Just a note on your posting about the Greek site with the PG. The site has been around for about three years, but they published only the core texts, the introductions, notes, Latin translations, addenda, corrigenda, etc, are not included.

The full edition is available only on CDs (in PDF and TIFF format) from different Chrisian organisations. Some of the volumes are also uploaded on Google Books.

I thought it had been around for awhile, but I've not mentioned it before. I noticed that it only contains the Greek texts; perhaps I should have pointed that out.