The Apology (Greek: Ἀπολογία Σωκράτους πρὸς τοὺς Δικαστάς) of Xenophon is an account of Socrates' defence at his trial in 399 BC. There are some differences of emphasis with Plato's dialogue of the same name, the only other extant account of the trial.
Free online Texts
Gutenberg: The Apology. Multiple formats.
Perseus: Greek text (Clarendon, 1921). English text (Heinemann, 1979). HTML and XML formats.
University of Adelaide (Internet Archive): The Apology of Xenophon, translated by H.G. Dakyns. Multiple formats.
Academia.edu: The Two Apologies: A Comparison of the Defense Speeches of Plato and Xenophon, by David Sage.
BBC Radio 4 In Our Time: Xenophon. Melvyn Bragg with Paul Cartledge, Edith Hall and Simon Goldhill.
History of Philosophy without any gaps: Socrates without Plato: the Accounts of Aristophanes and Xenophon - podcast.
Internet Encyclopedia of Philosophy: Xenophon.
University of Missouri-Kansas City School of Law: I.F. Stone Breaks the Socrates Story. 1979 New York Times Magazine interview.
Wikipedia: Apology (Xenophon).
The Great Conversation: Further Reading at Tom's Learning Notes
Plato: The Apology.
Aristophanes: The Clouds - The only contemporary account of Socrates extant, other than those of Plato and Xenophon.
Ancient Greek resources: Learn to read Greek classics in the original.