The Categories is the first book of Aristotle's Organon, the collection of writings which founded the discipline of logic, and were central to philosophical education for centuries. Setting out to enumerate all the possible kinds of things which can be the subject or predicate of a proposition, Aristotle comes up with the ten concepts that give the book its title: substance, quantity, quality, relation, place, time, attitude, condition, action and affection.
Free online and downloadable texts
Gutenberg: The Categories, translated by E.M. Edgehill. Multiple formats.
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.
Classical Library: The Categories, translated by E.M. Edgehill. HTML file.
Internet Classics Archive: Categories, translated by E.M. Edgehill. HTML and text files.
University of Adelaide (Internet Archive): The Categories, translated by E.M. Edgehill. EPUB, HTML and MOBI formats.
Wikisource: Categories, translated by O.F. Owen (1853).
History of Philosophy without any gaps: Aristotle's Logical Works - podcast by philosopher Peter Adamson.
Librivox: The Categories - public domain audiobook.
Stanford Encyclopedia of Philosophy: Aristotle's Categories, by Paul Studtmann.
Stanford Encyclopedia of Philosophy: Aristotle's Logic, by Robin Smith.
Wikipedia: The Categories (Aristotle).
The Great Conversation: Further reading at Tom's Learning Notes
Porphyry: Isagoge - an introduction to the Categories that strongly influenced the medieval reception of Aristotelian logic.
Ancient Greek resources: Learn to read Greek classics in the original.