The Isagoge or Introduction by Porphyry is a commentary on Aristotle's Categories, which itself became a key logical text of the Middle Ages, being translated into Arabic via Syriac, and into Latin by Boethius. Along with the Categories and On Interpretation, it formed part of the Ars Vetus or Old Logic, the works available in the Medieval Latin West prior to the translation of Aristotle's other logical works.
The medieval concept of the Porphyrian Tree was inspired by Porphyry's presentation of Aristotle's system of classification. Porphyry bracketed the issue of whether Aristotelian genera and species were merely concepts used to describe particular things or had independent reality, but his formulation of the question was, via Boethius, influential for the medieval debate about universals.
Free online texts
Forum Romanum: Isagoge, translatio Boethii. Latin text, HTML format.
Internet Archive: Porphyrii Isagoge et in Aristotelis Categorias commentarium, edited by Adolfus Busse (1887). Greek text, multiple formats.
The Logic Museum: Isagoge. Greek/Latin/English parallel text, HTML format.
Prometheus Trust: The Introduction of Porphyry to Aristotle's Categories, translated by Thomas Taylor. HTML format.
Tertullian.org: Introduction (or Isagoge) to the logical Categories of Aristotle, translated by Octavius Freire Owen (1853). HTML format. (see also the preface).
Universitatea Babeş-Bolya: Isagoge. Greek text, PDF format. Archived at the Internet Archive.
Universitatea Babeş-Bolya: Isagoge, translatio Boethii. Latin text, HTML format. Archived at the Internet Archive.
History of Philosophy without any gaps: King of Animals: Porphyry - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Neoplatonism.
Stanford Encyclopedia of Philosophy: Porphyry.
The Logic Museum: Isagoge.
Further reading at Tom's Learning Notes
The Enneads, Porphyry's account of the teachings of Plotinus.