The Pirkei D'Rav Eliezer says these were angels that fell from heaven and married the daughters of Kayin who were dressed inappropriately, and had children that were giants.
http://hebrewbooks.org/pdfpager.aspx?req=21858&st=&pgnum=131
http://hebrewbooks.org/pdfpager.aspx?req=21858&st=&pgnum=132