Utopia by Thomas More inaugurated a new literary genre on its first publication in Latin at Louvain, Belgium in 1516. It's title, from a Greek term meaning 'nowhere', has become the established term for any imaginary, ideal Commonwealth.
Utopia took inspiration from the voyages of discovery of More's own day, being ostensibly the account of Raphael Hythloday, a traveller with Amerigo Vespucci. In book 1, Hythloday and More discuss the role of the philosopher in civic life, touching on many of the ills of contemporary Europe. Hythloday introduces his account of the idealised society he encountered on the island of Utopia in book 2. The Utopian system of common property contrasts sharply with the enclosures then underway in contemporary England, and the sharpness of the book's satire may have contributed to its publication on the continent, under the editorship of Erasmus.
Free online texts
Bartleby: Utopia - Harvard Classics, Vol. 36, Part 3. HTML format.
Gutenberg: Utopia. Multiple formats.
Internet Archive: Utopia, Robinson translation and Latin text (1895). Multiple formats.
Marxists.org: Utopia. 1901 Cassell & Co. edition. HTML format.
Online Library of Liberty: Ideal Empires and Republics. Rousseau’s Social Contract, More’s Utopia, Bacon’s New Atlantis, Campanella’s City of the Sun, with an Introduction by Charles M. Andrews (1901). Multiple formats.
Open Utopia - 2016 translation, including letters, commendations and marginalia, with Creative Commons license. Multiple formats.
University of Adelaide (Internet Archive): Utopia. Multiple formats.