#7DayStory is run by Julie Duffy and Gabriela Pereira. 
I found this a great way to stimulate creativity and write a short story. Each day concentrates on one aspect. It moves forward until one has a final piece, polished and ready to send out thus proving that you can do it.
It is not a one off because I saved each prompt and I can use them time and again. If stuck, it’s a great way to achieve something and build confidence.
Because the first day focuses on building ideas before even attempting to write, you have a spark book with inspiration for the next time.
I wrote a ghost story and it is winging it’s way to a site called The Other Stories as I speak. Follow the link to see what the group does. http://www.theotherstories.org/
What an interesting idea. It sounds really useful.