1632: The Ring of Fire
Some of the greatest fun I've ever had as a writer has been to get to play in Eric Flint's New York Times-bestselling 1632 universe, which may well be the largest collaborative 'shared-world' project in SF history. It certainly seems to be one of the most economically profitable, and certainly the most broadly participatory.
My involvement in the Ring of Fire series began at Lunacon in 2009, when Eric invited me to make a submission for an upcoming 1632 anthology. That novella ultimately grew into part of an upcoming novel set in the New World. Shortly thereafter, Eric suggested a collaboration on a novel which is the sequel to 1635: Canon Law. The novel—which mixes the action of urban combat with the tension of political intrigue and assassination—is titled 1635: The Papal Stakes
But before that came to market, the 2011 anthology, Ring of Fire III arrived (apparently illustrated by the airship I designed...) with two of my stories in it: "Birds of a Feather" and "Upward Mobility."