On Proof And Progress In Mathematics (1994) [pdf]

Minor note: Thurston's 'geometrization conjecture', which Thurston talks about near the end of the essay, has since been proven by Grigori Perelman. This article was apparently written before then. http://en.wikipedia.org/wiki/Geometrization_conjecture

edit: article was published April 1994, Perelman's first paper appeared on the arxiv in November 2002