WARNING: The following websites are so full of writing prompts, collaborations, games and exercises, you might start writing whether you want to or not.
LANGUAGE IS A VIRUS
Language is a Virus.com has dozens of writing exercises, prompts, games and collaborative writing experiments. Going one step further, Language offers book publishing/selling support. This is a site that could easily threaten even the heartiest excuses for not writing and keep you busy for hours.