Internet History Sourcebooks Project
Fordham University: Medieval Sourcebook: Gregory of Nyssa: Life of Macrina
The complete text of Gregory's book on his sister, Macrina. The book was written during the 4th century.
Other
The Gnostic Society Library: The Acts of Peter and the Twelve Apostles
The complete text of the book from the new testament. It is one of the oldest copies available. It was found as part of the Nag Hammadi library.
New Advent
Catholic Encyclopedia: St. Gregory of Nyssa
A detailed biography of the Saint, it also provides a good summary of his major writings.