The community of Christian etext redactors is very active, and transcriptions of public domain books on the topic are abundant. The following are archives of public domain etexts about Christianity and related topics.
Some of these are: Christian Classics Ethereal Library This is probably the largest general purpose archive of Christian texts.
Additional Early Church Fathers at the Tertullian Project website. This dovetails with the 'Early Church Fathers' collection at CCEL.
The Unbound Bible has the best collection of downloadable Bible data files anywhere. Highly recommended.
Digital Christian Library and related topics, with many unique items, by a very active scanner.
The Gnostic Society A collection of shorter texts on Gnosticism. Also has a collection of Dead Sea Scroll translations.
The Catholic Encyclopeda This is a project to put online this huge work from 1913.
Project Wittenberg A comprehensive archive of Lutheran texts, of general interest for the study of Protestantism.
Quaker Heritage Press publishes etexts of rare and out-of-print Quaker documents.
Restoration Movement Texts. Primary texts of a 19th century American religious movement, predecessor of Churches of Christ, the Christian Churches, and the Disciples of Christ, among other groups.