Einstein's classic essay, 'Why Socialism?' appeared in Monthly Review, May 1949, and later in a collection of Einstein's writings -- Ideas and Opinions. It's online at http://www.monthlyreview.org/598einst.htm

. Helen Keller, "How I Became a Socialist": http://archive.8m.net/keller.htm

W.E.B. DuBois, in the American Socialist, January 1956: [Dead link removed. -Editor]

Also (though you probably wouldn't want to include this), DuBois' 'On Stalin', from the National Guardian, March 1953: http://www.northstarcompass.org/nsc0003/stalweb.htm