Top of the list

Christian Classics Ethereal Library (http://www.ccel.org)
Large collection of classics, especially the Fathers, and the Reformation writers.
Post-Reformation Digital Library (http://www.prdl.org)
Large collection of post-Reformation works (15th-18th c.).
Theological Commons (http://commons.ptsem.edu)
Large collection of classic works scanned at Princeton.
Early Church Texts (http://www.earlychurchtexts.com)
Extensive collection of patristic texts.
New Advent (http://www.newadvent.org/)
A Catholic site which offers Church Fathers, Thomas Aquinas, the Catholic Encyclopedia of 1912, and more.
Ecole Initiative (http://www2.evansville.edu/ecoleweb/)
Hyper-text encyclopedia of the early church.
The Vatican (http://www.vatican.va/)
The Vatican website, in multiple lanuages, offers many useful documents, including the texts of Vatican II.
Internet Medieval Sourcebook (http://www.fordham.edu/halsall/sbook.html)
Many links to medieval texts.
Internet Modern History Sourcebook (http://www.fordham.edu/halsall/sbook.html
Many links to historical texts.

Also very useful:

Crusades-Encyclopedia (http://www.crusades-encyclopedia.com)
Wide-ranging information on the crusades – both primary and secondary literature.
Dictionary of Methodism in Britain and Ireland (http://dmbi.wesleyhistoricalsociety.org.uk)
Online version of a book published by Epworth Press in 2000. Very useful.
Methodist Library Links (http://www.drew.edu/library/methodist/methodist-resources/links)
Large collection of links relevant for the study of Methodism.
e-Catena (http://www.earlychristianwritings.com/e-catena/)
This page offers the ability to search the early church fathers (“Ante-Nicene Fathers”) for quotations of the New Testament.
Saint Pachomius Library (http://www.voskrese.info/spl/index.html)
Links to texts from the Eastern Orthodox tradition.
AdHoc Image and Text Database on the History of Christianity (http://research.yale.edu:8084/divdl/adhoc/index.jsp)
A little slow to navigate through, but useful.
Digital Image Archive, Pitts Theological Library (http://www.pitts.emory.edu/dia/searchform.cfm)
A great site at site at the Candler School of Theology at Emory University in the USA. Offers images that can be searched by keyword or scripture reference.
James Kiefer’s Christian Biographies (http://elvis.rowan.edu/~kilroy/JEK/home.html)
Brief information on the Saints.
Tertullian Project (http://www.tertullian.org/)
Offers many text by and on Tertullian, and other church fathers.
Augustine of Hippo (http://ccat.sas.upenn.edu/jod/augustine.html)
Great site on Augustine.
All Catholic Church Ecumenical Councils – All the Decrees (http://www.piar.hu/councils/~index.htm)
Texts of all the ‘ecumenical councils’.
Guide to Early Church Documents (http://www.iclnet.org/pub/resources/christian-history.html)
Some links are no longer correct, but still a useful collection of links to early church texts.
Early Christian Writings (http://www.earlychristianwritings.com/)
Texts from the NT to Origen
EarlyChurch.Org.Uk (http://www.earlychurch.org.uk/about.html)
Early Church texts, links, and bibliographies.
Jewish Roots of Eastern Christian Mysticism (http://www.marquette.edu/maqom/)
Scholarship on … the Jewish Roots of Eastern Christian Mysticism.
Papal Encyclicals Online (http://www.papalencyclicals.net/)
A guide to papal and other important Roman Catholic church documents.
EWTN Virtual Library(http://www.ewtn.com/vlibrary/search.asp)
Good collection of texts relevant particular to the Roman Catholic tradition.
The Medieval Church (http://www.medievalchurch.org.uk/about.html)
Links to online texts, and bibliographies.
Benedictine Monasticism (http://www.faculty.de.gcsu.edu/~dvess/ids/medieval/benmon.html)
Resources for the study of medieval monasticism.
Project Wittenberg (http://www.iclnet.org/pub/resources/text/wittenberg/wittenberg-home.html)
Key Lutheran texts.
Gutenberg Digital (http://www.gutenbergdigital.de/gudi/eframes/index.htm)
Resources relating to the famous Gutenberg Bible!
Gutenberg Bible at the Ransom Center (http://www.hrc.utexas.edu/exhibitions/permanent/gutenberg/)
More digital resources on the Gutenberg Bible, this time from Texas.
Early English Books Online (http://eebo.chadwyck.com/home)
Includes a number of useful theological texts.
Gutenberg Project (http://promo.net/pg/)
This website has nothing to do with the Gutenberg Bible, but offers classic texts (some of theological-historical significance) online.
Great Books (http://anova.org/)
Similar site.
Oxford Text Archive (http://ota.ahds.ac.uk/)
Similar site, but with a British (rather than US-American) focus.
Websites for Mission Research (Henry Martyn Centre) (http://www.martynmission.cam.ac.uk/Research.htm)
Good collection of online links for the study of mission.

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