The Two Treatises of Government by John Locke are seminal works in the history of the liberal tradition, originally published in the aftermath of England's Whig revolution of 1688, with which Locke was intimately involved, though written some years earlier.
The First Treatise attacks the doctrine of the divine right of kings, defended by Anglican theologians such as Sir Robert Filmer, an important bulwark of the legitimacy of the Stuart monarchs.
The Second Treatise offers Locke's positive political theory, putting forward a social contract argument, which unlike that of Hobbes, provides a basis for limited representative government. If this Locke's key significance for his admirers, his critics point to his theory of property, which allows for slavery, and has been interpreted as allowing greater to rights to those who exploit natural resources more fully, potentially favouring European settlers in the Americas over natives.
Free online texts
Constitution.org: Second Treatise of Civil Government. HTML and TXT formats.
Early Modern Texts: Second Treatise on Government (1689), adapted and translated into modern English, by Jonathan Bennett. PDF format.
Gutenberg: Second Treatise of Government. Multiple formats.
Internet Archive: Two Treatises of Government, with Patriarcha by Robert Filmer, edited by Thomas I. Cook. Hafner Library of Classics. Multiple formats.
Marxists.org: The Second Treatise of Government. HTML format.
University of Adelaide (Internet Archive): The Second Treatise of Civil Government. Multiple formats.
Wikisource: Two Treatises of Government. HTML and other formats.
Librivox: Two Treatises of Civil Government - Public domain audiobook.
Philosophy - The Classics: Locke - 2nd Treatise. Podcast by Nigel Warburton.
PhilPapers: Locke - Two Treatise of Government - bibliography with open access option.
Further reading at Tom's Learning Notes
Sir Robert Filmer: Patriarcha - A defence of the divine right of kings. Locke's key polemic target in the first treatise.
Richard Hooker: Of the Laws of Ecclesiastical Politie.
Thomas Hobbes: Leviathan - Although it led to more authoritarian conclusions, Hobbes' social contract theory was an important influence on Locke's.