Traditionally ascribed to the sage Vyāsa, the poem deals with the Kurukshetra war between two sets of cousins, the Kauravas and the Pandavas. The historical kernel of these events is thought to date back to Vedic India around 1000 BCE, although the Mahabharata itself is thought to have been written down in the period from 400 BCE.
Embedded within the Mahabharata is the Bhagavad Gita, a philosophical dialogue between the Pandava hero Arjuna and his charioteer, the god Krishna, taking place before the onset of the climactic battle with the Kauravas.
Free online texts
Gutenberg: Mahabharata. K.M. Ganguli translation. Multiple formats.
Holy Books.com: The Mahabharata - K.M Ganguli translation, PDF format.
Internet Archive: The Mahabharata, translated by K.M. Ganguli (1884-96). Multiple formats.
Online Library of Liberty: The Ramayana and the Mahabharata, verse translation by Romesh C. Dutt. Multiple formats.
Sacred Texts: The Mahabharata, Sanskrit text and English translation by K.M. Ganguli. HTML format.
Wikisource: Sanskrit text and partial text of the English translation, by K.M Ganguli. HTML and other formats.
Performances and Reviews
New York Times: Peter Brook transforms an Indian epic for the stage, by Margaret Croyden, 25 August 1985.
Ancient History Encyclopedia: Mahabharata.
Brown University: The Mahabharata, pages by Sanskrit scholar James L. Fitzgerald.
History of Philosophy Without Any Gaps: Grand Illusion - Dharma and Deception in the Mahabharata, podcast episode by Peter Adamson and Jonardon Ganeri.
The Independent: The Book of a Lifetime - The Mahabharata, by Alice Albinia, 9 June 2011.
Librivox: The Mahabharata by Vyasa: The epic of ancient India condensed into English verse, public domain audiobook.
London Review of Books, How to Escape the Curse, John's Smith's translation reviewed by Wendy Doniger.
UC Davis: Mahabharata, synopsis by V. Vemuri.
The Great Conversation: Further reading at Tom's Learning Notes
Bloom's Western Canon - The Mahabharata is listed.