Parts of the PG can be found for free at the following sites:
http://www.documentacatholicaomnia.eu/
http://khazarzar.skeptik.net/pgm/
http://cyprianproject.info/PG.htm
http://classicsindex.wikispaces.com/migne_PG
http://ancientworldonline.blogspot.com/2009/10/migne-online.html