The Protagoras is a Socratic dialogue which features perhaps one of the most interesting of Socrates' intellectual opponents. Plato provides a relatively sympathetic portrait of the sophist Protagoras, a historical figure who is believed to have been close to the Athenian leader, Pericles.
Free Online and Downloadable Texts
About.com: Protagoras Dialogue - Benjamin Jowett Public Domain Translation.
Gutenberg: Protagoras by Plato, translated by Benjamin Jowett, multiple formats.
Loebulus. L165 - Plato - Laches. Protagoras. Meno. Euthydemus. PDF of public domain Loeb edition in Greek and English.
University of Adelaide (Internet Archive): Protagoras, translated by Benjamin Jowett. EPUB, HTML and MOBI formats.
Wikisource: Protagoras, translated by Benjamin Jowett.
Approaching Plato: A Guide to the Early and Middle Dialogues
Internet Encyclopedia of Philosophy: Protagoras - biography of the historical sophist.
PhilPapers: Plato - Protagoras - bibliography with open access option.
Librivox: Protagoras - audiobook.
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.