Of the fathers of the Church who seem to have favored universalist eschatologies, three in particular stand out to my mind:
Gregory of Nyssa (4th Century, Cappadocia)
Maximus the Confessor (6th-7th Centuries, Constintanople)
Isaac of Syria (7th Century, Ninevah)
Gregory of Nyssa explicitly teaches universal salvation in several works. He is sometimes referred to as "the flower of orthodoxy." It is said that Gregory of Nyssa, Gregory's brother Basil the Great, and their friend Gregory Nazianzus (the three "Cappadocian fathers") together form the Eastern equivalent of Augustine of Hippo in regards to their influence and major work. The three of them were especially important for developing the Trinitarian theology of the Eastern Church.
Maximus the Confessor is, with Gregory of Nyssa, canonized in both the Eastern Orthodox and Roman Catholic Churches (an impressive feat). Hans Urs von Balthasar expresses in his book "Dare We Hope" that his view is that Maximus also held to an eschatological universalism.
Finally, according to Eastern Orthodox blogger Alvin Kimel, Isaac of Syria is famous in the Orthodox Church for his universalism. According to Kimel, multiple Eastern Orthodox clergy have told him that St. Isaac convinced them that eschatological universalism is true.
I confess that I am not as versed in the writings and thought of these fathers as I would like, though I have read (at ccel.org) parts of translations of Gregory of Nyssa's "On the Soul and Resurrection," "The Making of Humanity," and "The Great Catechism" and have seen for myself multiple places where Gregory affirms not only the salvation of all human beings, but also the salvation of the Devil himself. For me, Gregory's belief in the salvation of demons is a welcome remedy to the Augustinian-Thomistic-Calvinistic pessimism regarding the possibility of the conversion of demons and the devil.
I wish I could say that I am intimate with the thought of Maximus and Isaac, but alas, I am not. However, I recommend two books on them -- which I am saving up my extra money for! -- that seem well-liked.
Hans Urs von Balthasar, a prominent Roman Catholic scholar and hopeful universalist (see "Dare We Hope"), wrote a book on Maximus the Confessor called "Cosmic Liturgy: The Universe According to Maximus the Confessor." von Balthasar allegedly discusses the question of Maximus' universalism in certain parts of the book.
Hilarion Alfeyev, a Russian Orthodox Bishop, authored a book on Isaac of Syria called "The Spiritual World of Isaac the Syrian." There is one chapter devoted to Isaac's eschatology. Again, I am really hoping to get this book soon.
I am not aware of specific titles on Gregory of Nyssa. However, the notable Eastern Orthodox philosopher (of Radical Orthodoxy) David Bentley Hart, devoted about 10-15 pages of his book "The Beauty of the Infinite: The Aesthetics of Christian Truth" to Gregory of Nyssa's eschatological universalism (in the second part of the chapter called "Eschaton"). Hart's treatment of Gregory of Nyssa is one of my favorite expositions of universalism that I have read thus far, along with "The Restoration of All Things" in pages 235-255 of the English translation of Jürgen Moltmann's "The Coming of God: Christian Eschatology" (since, after all, I do not read in German yet, sadly).
As an afterthought, another excellent treatment on eschatological universalism (also by David Bentley Hart) is his essay, "God, Creation, and Evil: The Moral Meaning of Creatio Ex Nihilo." It is only 17 pages. I highly recommend reading it! It can be accessed here: <http://journal.radicalorthodoxy.org/index.php/ROTPP/article/view/135/75>.
Finally, here is a link to the Internet Encyclopedia of Philosophy entry that is devoted to Gregory of Nyssa. It contains references to the works of Gregory of Nyssa that discuss his universalism. It can be accessed here: <http://www.iep.utm.edu/gregoryn/>.
My vision for this post is to become a place to discuss patristic universalism in general, but particularly for it to foster dialogue in general concerning these three major fathers of the Church. Please feel free to add to -- or debate! -- this post in any way!