Church Fathers by Alphabet. An alphabetical arrangement of the works of the early Christian writers.
Apostolic Fathers in various editions.
Hard-to-find translations of the Church Fathers.
English Renaissance translations of the Fathers from Early English Books Online.
Ebooks of the Church Fathers from Project Gutenberg.
A Library of Fathers of the Holy Catholic Church Anterior to the Division of the East and the West. Pusey’s project.
Ante-Nicene Christian Library.
Ante-Nicene, Nicene, and Post-Nicene Fathers, edited by Roberts, Donaldson, Schaff, and Wace.
Patrologiae Cursus Completus, edited by J.-P. Migne.
Maxima Bibliotheca Veterum Patrum, edited by Marguerin de La Bigne.
Eastern Christianity: Books from and about the Christian churches of the East that traditionally use languages other than Greek or Latin.
Ecclesiastical History up to the Dark Ages.
Christian History in the Middle Ages.
The Reformation and Antecedents.
The Jewish Encyclopedia is, of course, very useful to Christians.