On Friendship (Latin: Laelius de Amicitia) is a dialogue by Cicero, which argues that true friendship is founded on virtue. It was completed in 44 BCE and set in 129 BC in the period following the death of Scipio Aemilianus around 129 BC. The speakers are Scipio's friend Laelius, and his two sons-in-law, Fannius and Scaevola, the latter of whom taught Cicero law.
Free online texts
Gutenberg: Treatises on Friendship and Old Age by Marcus Tullius Cicero, translated by Evelyn Shuckburgh. Multiple formats.
LacusCurtius: Cicero on Friendship, translated by W.A. Falconer (1923). HTML format.
Latin Library: Laelius de Amicitia. Latin text, HTML format.
Perseus: De Amicitia. Latin text, HTML and XML formats.
Perseus: Laelius on Friendship, translated by W.A. Falconer (1923). HTML and XML format.
Wikisource: Laelius on Friendship. Latin text with facing English translation, multiple formats.
Wikisource: Laelius de Amicitia. 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.
The Great Conversation: Further reading at Tom's Learning Notes
Latin resources: Learn to read Latin texts in the original.