Thomas More-Utopia (Only 130 pages long, and the novel which invented the term 'Utopia'. Very influential.)
Plato-The Republic (Not exactly a novel, but still an important utopian text.)
Aldous Huxley-Island (Less well-known than Brave New World, this is his utopian novel.)
Charlotte Perkin Gilman-Herland etext.lib.virginia.edu/toc/modeng/public/GilHerl.html
Actually, theres an anthology book entitled 'The Faber Book Of Utopias', which is very good, covering utopias from Middle Kingdom Egypt to the present day. (Additionally, for some aspects of The Republic and Utopia, they're covered in my Extension 2 dissertation, in the 'exhibition' thread.)