The Metaphysics (Greek: τὰμετὰτὰφυσικά; Latin: Metaphysica) is a key philosophical work of Aristotle. The book's name originally came from its position after the Physics in Aristotle's collected writings. However, it's title subsequently provided the name for the branch of philosophy with which it was concerned, addressing the fundamental nature of being.
Free online texts
Bibliotheque Nationale De France: Aristotle's Metaphysics with Averroes' commentary. Latin text, image files.
The Classical Library: Metaphysics, translated by W.D. Ross. English HTML files.
Internet Archive: Organon and Other Works, translated by W.D. Ross. Multiple formats.
Internet Classics Archive: Metaphysics, translated by W.D. Ross. English HTML and TXT files.
University of Adelaide (Internet Archive): Metaphysics, translated by W.D. Ross. EPUB, HTML and MOBI formats.
History of Philosophy without any gaps: Down to Earth Aristotle on Substance - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: God Only Knows: Aristotle on Mind and God - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Aristotle - Metaphysics, by Joe Sachs.
Librivox: Metaphysics - public domain audiobook.
Stanford Encyclopedia of Philosophy: Aristotle's Metaphysics, by S. Marc Cohen.
University of Washington: An Outline of Metaphysics Ζ, by S. Marc Cohen.
Wikipedia: Metaphysics (Aristotle).
The Great Conversation: Related reading at Tom's Learning Notes.
Plato: The Republic - key text for the theory of the forms.
Ancient Greek resources: Learn to read Greek classics in the original.