History

Aristotle's Logic
Introductory article by Garth Kemerling.
http://www.philosophypages.com/hy/2n.htm

Aristotle's Reform of Paideia
Article by Evelyn Barker, arguing that the principal aim of Aristotle's Organon was to reform the contemporary paedagogical role of dialectic.
http://www.bu.edu/wcp/Papers/Anci/AnciBark.htm

Logic
Entry in the Catholic Encyclopaedia (1917), dominated by a historical survey from Indian and Pre-Aristotelian philosophy to the Logic of John Stuart Mill.
http://www.newadvent.org/cathen/09324a.htm

Mediaeval Logic and Philosophy
Resource maintained by Paul Vincent Spade at Indiana University.
http://pvspade.com/Logic/

Medieval Theories of Analogy
Entry in the Stanford Encyclopaedia of Philosophy, by E. Jennifer Ashworth.
http://plato.stanford.edu/entries/analogy-medieval/

Medieval Theories of Modality
Article at the Stanford Encyclopaedia of Philosophy.
http://plato.stanford.edu/entries/modality-medieval/

On Dialectic
By St. Augustine. Latin original together with translation by J. Marchand.
http://www9.georgetown.edu/faculty/jod/texts/dialecticatrans.html

Peter of Spain (Petrus Hispanus)
Life and work of 13th century logician and author of the Tractatus; from the Stanford Encyclopedia by Joke Spruyt.
http://plato.stanford.edu/entries/peter-spain/

Scholastic Logic
Chapter Two of Joseph Perrier's "The Revival of Scholastic Philosophy in the Nineteenth Century" (1909).
http://www.nd.edu/Departments/Maritain/etext/perrier2.html

Scholasticism
Online text by Joseph Rickaby (1908).
http://www.nd.edu/Departments/Maritain/etext/scholas1.htm

Square of Opposition
Article in the Stanford Encyclopaedia of Philosophy, by Terence Parsons.
http://plato.stanford.edu/entries/square/

The Summulae of John Buridan
A critical introduction by Gyula Klima.
http://www.fordham.edu/gsas/phil/klima/Intro.htm

Syllogistic logic online
This site contains an online program for studying Aristotle's syllogistic logic. The theory of Aristotle's ancient logic is also presented.
http://aristotelianlogic.glashoff.net