Selected sites are listed below; most of these will lead to many further sites of potential interest and/or usefulness.
Byzantium: Byzantine Studies on the Internet.
The focus is on Byzantine studies, naturally, but contains much that is of interest for the medieval church historian. Contents include information on the annual Byzantine Studies conference, course syllabi and outlines, shareware available over the Internet, bibliographies, full texts, images, articles, book reviews, list of discussion lists relevant for Byzantine studies and many links to other web resources.
The Labyrinth: A World Wide Web Server for Medieval Studies.
A pioneering web site for medieval studies, as of 2021, archived and no longer updated. There are sections devoted to Philosophy, Theology and Church History but the historian of medieval church history will find much of potential relevance in other sections as well.
Online Medieval Sources Bibliography
This site provides a searchable bibliographic database of those medieval texts (including both non-literary texts such as letters, wills, etc. and literary texts) now available in printed or online editions and translations. The entries in the bibliography are annotated with information on genre, contents, original language, and information about the published version. Entries also include links to sources that are online.
Online Reference Book for Medieval Studies.
An effort by medievalists to produce an "online textbook source for medieval studies on the World Wide Web." The site includes an "encyclopedia" of original essays (including several on religion), resources for teaching, bibliographies, and an especially valuable, alphabetized (by author) list of full-text ancient and medieval sources, the ORB Library Connection.
Internet Medieval Sourcebook.
Located a Fordham University, this is another good site for access to full-text. There is a section of "Selected Sources" designed to be used as a teaching aid, a section of "Full Text Sources," a listing of sources by type, and a third section on "Saints Lives."