The Non-canonical Christian Texts pages provide links to extensive resources on non-canonical texts related to the New Testament. The Gospel of Thomas has its own page because of the extensive resources available on this. For information on the the formation of the canon, go to the page on Canon.
A site with a series of texts including apocryphal NT Gospels, Acts, apocalypses and more. By using older translations that are now out of copyright, this site is able to make available a lot of material. The site is now beginning to get a bit long in the tooth — it has not been updated since 2006.
A remarkable resource with lots of texts in English translation, focusing specially on Nag Hammadi. The Gnosis Archive is also searchable.
Gnostische Spiritualität im frühen Christentum, by Gerd Lüdemann & Martina Janßen (Stuttgart: Radius-Verl, 1997): full German text of the book of Gnostic prayers, hymns and other material from the beginnings of Christianity.
Developed in association with the Christian Apocrypha Section of the Society of Biblical Literature: a good jumping-off point with a handful of useful links.
By Andrew Bernhard: this fine web resource features introductions, fresh translations, bibliography and links on canonical, non-canonical and hypothetical gospels. Strongly recommended.
By István Czachesz: web resource focussing on the Apocryphal Acts, with special reference to the Acts of Paul, the Acts of Peter, the Acts of John, the Acts of Andrew, and the Acts of Thomas. A growing resources currently featuring links and bibliography.
By István Czachesz: full text in PDF files of a book forthcoming from Peeters (2002).
By Wieland Willker: useful material in both English and German. It includes photographs from Morton Smith’s book and now the full Greek text.
A remarkable resource also from Wieland Willker featuring a wealth of useful information on the fragments of a lost gospel known as Papyrus Egerton 2 — photographs, the texts in Greek with reconstructions, German and English translations, comprehensive bibliography and the full text of Bell and Skeat.
Analysis of the Canonical and Apocryphal New Testament Scriptures, by Geoff Trowbridge: bibliography and good introductions to many non-canonical texts.
The full text of the volume of the Early Church Fathers series that deals with the Gospel of Peter, Tatian’s Diatessaron, Apocalypses and Romances, and the Epistles of Clement.
By James Akin: useful list of possible references to texts from Apocryphal books in the New Testament.
From Pitts Theology Library at Emory University: a useful guide to reliable (print) literature on early Christian apocrypha, including a helpful list of non-canonical texts with brief descriptions and references to respected translations.