The Organon is the traditional collection of Aristotle's logical writings. It formed the basis of philosophical education for much of the ancient and medieval periods. It includes the following works:
Free online texts
Internet Archive: L 325 - The Categories and On Interpretation, translated by Harold P. Cook. Prior Analytics, translated by Hugh Tredennick. Greek and English Loeb edition. Multiple formats.
Internet Archive: L391 - Posterior Analytics & Topica. Greek and English Loeb edition.
Internet Archive: Organon and Other Works, translated under the editorship of W.D. Ross. Multiple formats.
Loebulus. L400 - Aristotle -- On Sophistical Refutations. On Coming-to-be and Passing Away. On the Cosmos. PDF of public domain Loeb edition in Greek and English.
Wikisource: The Organon, or Logical Treatises of Aristotle, with the Introduction by Porphyry, translated by Octavius Freire Owen (1863).
History of Philosophy without any gaps: Aristotle's Logical Works - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Hugh Benson on Aristotelian Method- podcast by philosopher Peter Adamson.
Stanford Encyclopedia of Philosophy: Aristotle's Logic, by Robin Smith.
Stanford Encyclopedia of Philosophy: The Traditional Square of Opposition, by Terence Parsons.
The First Science: Online Syllogism Solver.
Wikipedia: The Organon.
The Great Conversation: Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.