Aquinas

Aquinas, O.P., Thomas; Saint, Doctor [1225 - 1274]

Catechetical Instructions (wiki) - A short Catechism of the Catholic Church; providing a discussion of the Ten Commandments, The Lord's Prayer, The Sacraments of the Catholic Church, and the Hail Mary.
Catechetical Instructions (html)
Catechetical Instructions (pdf)
Summa Theologica (html) - Perhaps the most definitive work on the subjects of Theology and Faith, the Summa Theologica has profoundly impacted Christian thought and philosophy; not only Catholic, but protestant as well.
Companion to the Summa - Walter Farrell, O.P. (html)
The Summa Contra Gentiles of St. Thomas Aquinas (Abridged) (pdf) - (Of God and His Creatures: an Annotated Translation by Fr. Rickaby, S.J.)
The Summa Contra Gentiles, Book 1 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 2 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 3, Volume 1 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 3, Volume 2 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles, Book 4 (Unabridged) (.pdf) - (Translated by the English Dominican Fathers)
The Summa Contra Gentiles of St. Thomas Aquinas (Unabridged) (wiki) - The English Dominican Fathers
Catena Aurea: The Gospel of Matthew (pdf) - A commentary on the Gospel by St. Thomas Aquinas
Catena Aurea: The Gospel of Matthew (Revised) (pdf) - A revised version with improved navigation (5.5 MB)
Catena Aurea: The Gospel of Matthew (html) - Online version in a chapter format
Catena Aurea: The Gospel of Mark (pdf) - A commentary on the Gospel by St. Thomas Aquinas
De Aeternitate Mundi (pdf) - "On the Eternity of the World" by St. Thomas Aquinas
De Aeternitate Mundi (wiki)
De Ente et Essentia (pdf)- "On Being and Essence" by St. Thomas Aquinas
De Ente et Essentia (wiki)