Significant portions of two magisterial series of critical editions for Christian texts from Late Antiquity can be easily downloaded from enumerated lists linked to archive.org and Google Books: Die Griechischen Christlichen Schriftsteller (GCS), available at Roger Pearse and Patrologia Latina, Graeca, & Orientalis (PLGO; through Scribd); and Corpus Scriptorum Ecclesiasticorum Latinorum (CSEL), also available at Roger Pearse and the PLGO. While all of the texts from the series Sources Chrétiennes, founded in 1942, are still under copyright, useful information on the many volumes can be found on the Institute’s website. Similarly, Brepols Corpus Christianorum, and its various subseries, are under copyright; the Series Latina is available by subscription in the Library of Latin Texts.
This site offers basic information on the Old Latin (Vetus Latina) versions of the bible, which remain comparatively intractable and overlooked in research on the history of the biblical text and of early Christianity. Last updated in 2008, it nevertheless contains some useful information, including a book-by-book list of the available editions, an ongoing project of the Institut Vetus Latina in Beuron, published by Herder Press. Also useful is the explanation of the numbering system for Old Latin manuscripts, which includes a few stray images. To my knowledge, the only fully digitized Old Latin manuscript is Codex Bezae at Cambridge; some important codices, such as Codex Veronensis (4), are partly photographed as part of the Verbum Project, which I will review shortly. The extensive note cards of patristic citations, held at the Institut Vetus Latina, have been digitized and are available for a subscription fee from Brepols, but even this resource can be difficult to navigate.