The Monadology (French: La Monadologie) is a short 1714 text by Leibniz outlining his metaphysics in ninety theses. Opposing the mind-body dualism of Descartes, Leibniz proposed a monistic idealist system in which the universe is made up of simple parts known as monads. These parts cannot directly affect each other. Rather reality is the result of a pre-established harmony between them.
Free online texts
Early Modern Texts: Monadology, adapted and translated into modern English, by Jonathan Bennett. PDF format.
Gutenberg: Leibnitz' Monadologie. German text, multiple formats.
Internet Archive: The Monadology and Other Philosophical Writings, translated by Robert Latta (1898). Multiple formats.
University of Adelaide (Internet Archive): The Monadology, translated by Robert Latta. Multiple formats.
University of California, San Diego (Internet Archive): Monadology, translated by Robert Latta, revised by Donald Rutherford. HTML format.
University of Leeds (Internet Archive): Monadology, translated by George Macdonald Ross (1999). Multiple formats.
Librivox: The Monadology - public domain audiobook.
PhilPapers: Gottfried Wilhelm Leibniz - bibliography with open access option.
Further reading at Tom's Learning Notes
Voltaire: Candide - a famous satire on Leibniz's optimism.