Guide to Early Church Documents
At ICLNet (Internet Christian Library): this site provides pointers to lots of material on the early church, including "canonical documents, creeds, the writings of the Apostolic Fathers and other historical texts relevant to church history". This is a good place to begin internet research on Patristics.

Christian Classics Ethereal Library
Based at Calvin College, U.S.A.: a treasure trove of resources -- features the whole of the famous 38-volume collection of writings from the first 800 years of the Church, either for viewing on the web or for downloading onto your own machine. Also features the World Wide Study Bible

Early Christian Writings
By Peter Kirby: very useful guide to Christian literature of the first two centuries, "The New Testament, Apocrypha, Gnostics, and Church Fathers" arranged chronologically. Sections on each writing or set of writings features links to on-line versions of the primary texts, links to secondary material and often some useful introductory material compiled by Kirby himself. An ambitious project that is already a fine guide for students of the New Testament.

Apostolic Fathers Look Up Tool (New)
By Rick Brannan: very useful tool enabling you to look up an Apostolic Fathers citation, in Greek (unicode) and English. There are more details on ricoblog.

The Ecole Initiative
An exciting and valuable project to create "a hyper-text encyclopedia of early Church history on the world wide web".

Earlychurch.org.uk
By Robert Bradshaw: comprehensive resource featuring a lot of bibliography and some full text article reproductions.

The Centre for Marcionite Research
By Daniel J. Mahar: useful material on the second century heretic Marcion, including reproductions of old reconstructions of Marcion's Gospel of the Lord and excerpts from Adolf Von Harnack's History of Dogma.

Egeria and The Fourth Century Liturgy of Jerusalem
By Michael Fraser: a hypertext version of the Latin text and English translation.

Augustine of Hippo
By James O'Donnell: a terrific resource featuring texts, essays and more; one of the oldest sites on the internet relating to early Christianity and still one of the best.