The Polish Virtual Library of Science made quite a few classical volumes available. Among them, some historical volumes of Acta Arithmetica, Annales Polonici Mathematici, Fundamenta Mathematicae, Mathematical Monographs series, Mathematical Dissertations and Studia Mathematica. Here. Of special interest, Kuratowski's and Sierpinski's works in the Mathematical Monographs series. Here. Alas, some of the stuff is in French and some is in Polish.