The Topics (Greek: Τοπικά; Latin: Topica) is the fifth work in the traditional collection of Aristotle's logical writings, the Organon. Whereas the Posterior Analytics is concerned with scientific demonstration based on true premises, the Topics focuses on dialectic argument based on premises which are merely agreed by common consent to be true. It's subject matter therefore has a large overlap with that of rhetoric.
Free online texts
Internet Archive: L391 - Posterior Analytics & Topica. Greek and English Loeb edition. Multiple formats.
Internet Classics Archive: Topics. HTML and text files.
University of Adelaide (Internet Archive): Topics, Translated by W. A. Pickard-Cambridge. Multiple formats.
Wikisource: Topics, translated by O.F. Owen (1853). Multiple formats.
History of Philosophy without any gaps: Aristotle's Logical Works - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Aristotle: Logic, by Louis F. Groarke.
Librivox: Topics - public domain audiobook.
Stanford Encyclopedia of Philosophy: Aristotle's Logic, by Robin Smith.
Wikipedia: Topics (Aristotle).
Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.