Ante-Nicene Fathers - Wikisource, the free online library
Nicene and Post-Nicene Fathers: Series I - Wikisource, the free online library
Nicene and Post-Nicene Fathers: Series II - Wikisource, the free online library
Early Church Fathers (tertullian.org)
Writings of the Early Church Fathers (biblestudytools.com)
Early Church Fathers - Christian Classics Ethereal Library (ccel.org)