Mission of Gravity Q6: Open source science fiction: Whirligig world
In the afterworld, Clement gives "official permission to anyone who so desires to lay scenes [on Mesklin.] I ask only that he maintain reasonable scientific standards, and that’s certainly an elastic requirement in the field of science fiction. "
Certainly there have been other shared worlds, but this is a very early (if not the earliest) example of an author offering up such a shared space. Heavy Planet collects a few other of these stories.
Could you write a story set on Mesklin? Would you read another story set on this planet?