The "Anonymous" Sayings of the Desert Fathers 
The Tales and Sayings of the Desert Fathers (Apophthegmata Patrum) are a key source of evidence for the practice and theory respectively of eremitic monasticism, a significant phenomenon within the early history of Christianity. The publication of this book finally ensures the availability of all three major collections which constitute the work, edited and translated into English. The collection presented in this volume furnishes almost as much material for the study of the late antique world from which the monk sought to escape as it does for the monastic endeavor itself. More material continued to be added well into the seventh century, and so the spread and gradual evolution of monasticism are illustrated here over a period of about two and a half centuries. Open-access version is available through Internet Archive: click on the unlocked icon.
Decrees of the Ecumenical Councils 
Edited by Norman Tanner, this monumental two-volume publication brings together in the original languages and an English translation all the decrees of all the ecumenical councils from Nicaea I to Vatican II. As such it represents an indispensable reference work for theologians and historians, as well as for anyone interested in the development of Christian thought and doctrine.
The Christian Classics Ethereal Library provides open access to the essential corpus of both the Greek and Latin Fathers in English translation.
Provides almost 2,000 Greek and Latin classic works translated into English by recognized scholars. The English translation appears with the original Greek or Latin on the opposite page. New titles are added several times a year. Search by author, title, editor, translator, period, or keyword or browse by author, Greek titles, or Latin titles. While the great majority of works in this collection are by pagan authors (including writers such as Aristotle, Cicero, Plato, and others who had enormous influence on later Christian thinkers), it includes several volumes by both the Latin and Greek Church Fathers.
Papal Encyclicals Online: Church Councils
This site, which provides open-access translations of papal encyclicals and other documents, also offers English translations of the canons of all the ecumenical councils of the Church starting with the First Council of Nicaea in 325--a very handy online reference, though Tanner's Decrees of the Ecumenical Councils should be used instead in any formal citation.
Published online by Villanova University, this translation is by Robert Russell, OSA., and is based on the critical text of Luc Verheijen, OSA, (La regle de saint Augustin, Etudes Augustiniennes, Paris, 1967).
The 1949 edition translated by Boniface Verheyen, OSB. Opens with a table of contents with each chapter getting its own link.
Decrees of the Ecumenical Councils
Edited by Norman Tanner, this monumental two-volume publication brings together in the original languages and an English translation all the decrees of all the ecumenical councils from Nicaea I to Vatican II. As such it represents an indispensable reference work for theologians and historians, as well as for anyone interested in the development of Christian thought and doctrine.
Provides almost 2,000 Greek and Latin classic works translated into English by recognized scholars. The English translation appears with the original Greek or Latin on the opposite page. New titles are added several times a year. Search by author, title, editor, translator, period, or keyword or browse by author, Greek titles, or Latin titles. While the great majority of works in this collection are by pagan authors (including writers such as Aristotle, Cicero, Plato, and others who had enormous influence on later Christian thinkers), it includes several volumes by both the Latin and Greek Church Fathers.
An electronic version of the first edition of Jacques-Paul Migne's Patrologia Latina, published between 1844 and 1855, and the four volumes of indexes published between 1862 and 1865. The Patrologia Latina comprises the works of the Church Fathers from Tertullian in 200 AD to the death of Pope Innocent III in 1216.
Began in 1972, the Thesaurus Linguae Graecae has collected and digitized most texts written in Greek from Homer to the fall of Byzantium in 1453. Its goal is to create a comprehensive digital library of Greek literature from antiquity to the present era.