Lucretius' On the Nature of Things (Latin: De Rerum Natura) is an epic poem dramatising an epicurean, materialist view of the world. As such, it is unique in Latin literature, as the later Roman reception of Greek philosophy was heavily influenced by Cicero's rejection of epicureanism.
Free online texts
Gutenberg: On the Nature of Things, translated by William Ellery Leonard. Multiple formats.
Internet Archive: De Rerum Natura Libri Sex. Latin Tuebner edition (1874). multiple formats.
Internet Classics Archive: On the Nature of Things, translated by William Ellery Leonard. html and txt files.
University of Adelaide (Internet Archive): Of the Nature of Things, translated by William Ellery Leonard. EPUB, HTML and MOBI formats.
Wikisource: On the Nature of Things, translated by William Ellery Leonard (1916). html.
BBC Radio 4 In Our Time: Epicureanism. Melvyn Bragg with Angie Hobbs, David Sedley and James Warren.
History of Philosophy without any gaps: Reaping the Harvest: Lucretius - podcast by philosopher Peter Adamson.
Literature and History: Episode 45 - The Uncuttables, Lucretius' On the Nature of Things and Epicurean Philosophy. Podcast and transcript by Doug Metzger.
Librivox: Lucretius - public domain audiobooks in English, Latin and French.
1911 Encyclopedia Britannica: Lucretius.
Gutenberg: Three Philosophical Poets - Lucretius, Dante and Goethe, by George Santayana. Multiple formats.
Internet Archive: Lucretius, by W.H. Mallock (1898).
Internet Encyclopedia of Philosophy: Lucretius.
Leeds International Classical Studies, Volume 1. Peer-reviewed articles on Lucretius.
PhilPapers: Lucretius - bibliography with open access option.
Stanford Encyclopedia of Philosophy: Lucretius, by David Sedley.
The Great Conversation: Further reading at Tom's Learning Notes
Latin resources: Learn to read Latin texts in the original.
Virgil: The Georgics.
Bloom's Western Canon: On the Nature of Things is listed.