The Tusculan Disputations (Latin: Tusculanae Disputationes or Tusculanae Quaestiones), written in 44BC, is a philosophical treatise in which Cicero defends Stoic views on happiness. The opening dedication to Brutus defends the aspiration for a Latin philosophical literature that could surpass the Greeks. The following five books portray a series of Socratic debates said to have taken place at Cicero's villa in Tusculum. Book 1 deals with the nature of death, book 2 with physical suffering, books 3 and 4 with mental suffering, while book 5 propounds the Stoic view that virtue is always sufficient for happiness.
Free online texts
Gutenberg: Cicero's Tusculan Disputations. Multiple formats.
Gutenberg: The Academic Questions, Treatise De Finibus, and Tusculan Disputations. Multiple formats.
Internet Archive: Cicero's Tusculan Disputations : also treatises On the Nature of the Gods, and On the Commonwealth, translated by C.D. Yonge (1877). Multiple formats.
Internet Archive: Cicero's Tusculan Disputations, translated by A.P. Peabody (1886). Multiple formats.
Latin Library: Tusculanae Disputationes. Latin text, HTML format.
Perseus: Tusculanae Disputationes (Teubner, 1918). Latin text, HTML and XML format.
University of Adelaide (Internet Archive): The Tusculan Disputations, translated by Charles Duke Yonge. Multiple formats.
Wikisource: Tusculanae Disputationes. Latin text, multiple formats.
History of Philosophy without any gaps: Rhetorical Questions: Cicero - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Raphael Woolf on Cicero - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Cicero.
Librivox: Tusculan Disputations - public domain audiobook.
Tusculan Disputations - chapter summaries, by John Uebersax.
The Great Conversation: Further reading at Tom's Learning Notes
Cicero: De Finibus.
Latin resources: Learn to read Latin texts in the original.