On the Soul (Greek Περὶ Ψυχῆς, Perì Psūchês; Latin De Anima) is a work of Aristotle which examines the nature of the soul and its relation to the body, which Aristotle takes to be an instance of his more general distinction between matter and form.
Free online texts
Internet Archive: De Anima, translated by R.D. Hicks, (Cambridge, 1906). Greek and English text. Multiple formats.
Internet Archive: Peri Psyches, translated by Edwin Wallace (Cambridge, 1882). Greek and English text. Multiple formats.
Internet Classics Archive: On the Soul, translated by J.A. Smith. HTML and TXT formats.
Classics in the History of Psychology: De Anima (On the Soul), translated by J.A. Smith. HTML format.
University of Adelaide (Internet Archive): On the Soul, translated by J.A. Smith. Multiple formats.
Wikisource: On the Vital Principle, translated by Charles Collier (1855). Multiple formats.
History of Philosophy without any gaps: Soul Power: Aristotle's De Anima - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: God Only Knows: Aristotle on Mind and God - podcast by philosopher Peter Adamson.
Librivox: De Anima - public domain audiobook.
Stanford Encyclopedia of Philosophy: Ancient Theories of the Soul, by Hendrik Lorenz
Stanford Encyclopedia of Philosophy: Aristotle's Psychology, by Christopher Shields.
Wikipedia: On The Soul.
The Great Conversation: Further reading at Tom's Learning Notes.
Ancient Greek resources: Learn to read Greek classics in the original.