Helping students with imaginative writing

There are many apps and websites which help students to get some ideas for their creative writing. Students also get to see the stories written by others or get feedback on their own. 

I have had a look at Plinky which seems interesting. There's also Boomwriter which looks very professional, but they are wanting to sell you things (though your class' own book of stories might be quite cool).

Teachers, please let me know if you have a go with any of them and what you think.

Steve