The Republic is Plato's best-known work. An extended reflection on the nature of justice, which retains much of the style of the early dialogues in the opening conversation in Athens, pitting Socrates against the cynical sophist Thrasymachus. From there the dialogue opens out into an extended discussion of the politics of the ideal city, which is closely bound up with ethical considerations, before taking in metaphysical and theological themes with Plato's most striking metaphor, the parable of the cave, and a meditation on the the fate of the individual soul in the myth of Er.
Free Online and Downloadable Texts
Gutenberg: The Republic by Plato, translated by Benjamin Jowett, multiple formats.
Internet Classics Archive: The Republic translated by Benjamin Jowett, online text.
Libertyfund: The Republic, translated by Benjamin Jowett, online text.
University of Adelaide (Internet Archive): The Republic, translated by Benjamin Jowett. EPUB, HTML and MOBI formats.
Wikisource: The Republic, translated by Benjamin Jowett. Online text.
BBC Radio 4 In Our Time: Plato's Republic. Melvyn Bragg with Angie Hobbs, MM McCabe, James Warren.
History of Philosophy without any gaps: Soul and the City: Plato's Political Philosophy - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Ain't No Sunshine: The Cave Allegory of Plato's Republic - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Plato: The Republic.
Librivox: The Republic - audiobook.
Openculture: Orson Welles Narrates an Animation of Plato’s Cave Allegory - video.
Open Culture: Plato’s Cave Allegory Brought to Life with Claymation - video.
PhilPapers: Plato - Republic - bibliography with open source option.
Stanford Encyclopedia of Philosophy: Plato's Ethics and Politics in the Republic.
Stanford Encyclopedia of Philosophy: Callicles and Thrasymachus.
University of Utah: Culture and Society in Plato's Republic, by M.F. Burnyeat. PDF format.
Wikipedia: The Republic (Plato).
The Great Conversation: Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.
Bloom's Western Canon: Plato's Dialogues are listed.