The Brown University website features a fairytale generator that allows you to select certain folk themes (such as 'villainy,' 'rescue,' 'struggle,' and 'guidance') and folk story presets (such as Cinderella, Hansel and Gretel, and Snow White), and the generator produces a unique story.  This concept is based on the theories of Russian scholar Vladimir Propp.

This is a fun page that kids will enjoy.  This tool allows you to select certain basic fairytales, like The Gingerbread Man and Little Red Riding Hood, and it prompts you to fill in the blanks of the story with the words of your choosing.  Simply click on your word and drag it into the text of the story.