I recently discovered The First Line Literary Magazine, which has a fascinating premise. The call for submissions for each issue gives you the first line of the story, and then you take it from there, the idea being that different authors will have very different takes on where to go with it. So fun!
I submitted a new story in my "Ransom and the ..." series in response to "The Window was open just enough to let in the cool night air" and "Ransom and the Open Window" was one of eight stories selected out of the nearly 1,000 submissions. Pretty pleased with that, given it was my first try at this game. If you're looking for a stimulus to get you past writer's block or just want a fun challenge, one could do worse than giving this contest series a try. It's a paying market, but the fun quotient far outweighs the token payment.