Alex Keegan's Bootcamp
Alex Keegan's Bootcamp is a tough online creative writing group. I've never been a member but I know some who have, and I know that opinion is strictly divided between those who love the place and those who hate it. There is no middle ground.
Bootcamp used to run an event, open to outsiders, to raise money for BBC Children in Need. This involved writing a new prompt-based piece every hour as often as possible over a forty-eight hour period. This is still one of the most insanely creative writing experiences I've ever been involved in, resulting in several useable stories.
They also occasionally run a month-long Blast, also open to outsiders, which involves writing a new piece in an hour every day based on prompts.