The Summa Theologica or Summa Theologiae by Saint Thomas Aquinas (1225-1274) is one of the best known philosophical works of the Middle Ages. Intended as a comprehensive guide to theology for beginning students, the first part of the work deals with God, nature and man, the second part with law and morality, while the third, unfinished part deals with Christ and the sacraments, seen as the route of humanity's return to God, thus giving the whole a cyclical structure.
Free online texts
Christian Classics Ethereal Library: Summa Theologica, translated by Fathers of the English Dominican Province (1947). Multiple formats.
Corpus Thomisticum: Summa Theologiae - Latin text. HTML format.
Google Play: STh lt - App containing the text of the Summa from the Corpus Thomisticum Project.
Internet Archive: Summa Theologica, Latin text (1894). Multiple formats.
Intratext: Summa Theologica, English translation. HTML format.
New Advent: The Summa Theologiæ of St. Thomas Aquinas, translated by Fathers of the English Dominican Province (1920). HTML format.
Sacred Texts: Summa Theologica, translated by the Fathers of the English Dominican Province (1947). HTML format.
SummaTheologica.info: Summa Theologica, English translation with onsite Google search. HTML and PDF formats.
University of Notre Dame: Summa Theologica, ongoing translation by Alfred J. Freddoso. PDF format.
BBC Radio 4 In Our Time: St Thomas Aquinas. Melvyn Bragg with Martin Palmer, John Haldane and Annabel Brett.
History Of Philosophy Without Any Gaps: 243 The Ox Heard Round the World - Thomas Aquinas | 244 Everybody Needs Some Body: Aquinas on Soul and Knowledge | 248 - Scott MacDonald on Aquinas, podcast by Peter Adamson.
Internet Encyclopedia of Philosophy: Thomas Aquinas.
Librivox: Summa Theologica, public domain audiobook.
Stanford Encyclopedia of Philosophy: Saint Thomas Aquinas.
Wikipedia: Summa Theologica.
Further reading at Tom's Learning Notes
Aristotle, referred to by Aquinas as 'The Philosopher': Metaphysics, Ethics.
St Paul 'the Apostle'
Dionysius the Areopagite
Augustine 'the Theologian'
Ulpian 'the Jurist'
Averroes 'the Commentator'
Hugo of St Victor
Peter the Lombard: The Sentences.
Latin Resources: Online materials for learning Latin.