I've been a member of a micro/flash fiction writing group on Yahoo! for probably close to a year now. The moderator offers up one word writing prompts that can be written in any genre, style, etc. up to 1,000 words (sometimes slightly longer works are allowed) but the prompt word, or its o. It is very open ended and can lead to some interesting takes. Some or all reply with their offerings then whomever feels like it, offers a critique of the stories. Literary writing is not welcomed.
On Google+, I've found a fellow writer offering up writing prompts. In the ones I've worked on there has been a premise to what you're supposed to be writing, with a word count limit. The prompts are fairly detailed, but yet open-ended at the same time. Today's, which I'll be posting in a few, was the first line of a Ken Follet book. The people who have posted each had a different take on the line. There is no critique per se, but the others let you know their reactions to the stories. Literary writing is welcomed.
Between the two, I have found myself being much more driven to write using the more detailed prompts. There are more rules and requirements but it really focuses my thoughts as I'm writing. I am going to be leaving the first group as I want to write more literary style stuff. I have learned much from them but it is time to spread my wings and see where else I can learn more.