Maybe someone else can find it, but scottaaronson.com has an article that describes a scheme where there's a cheap computation that has to be done serially, so

Maybe someone else can find it, but scottaaronson.com has an article that describes a scheme where there's a cheap computation that has to be done serially, so anyone can open if it they crunch cheap hashes for the requisit amount of time. The cleverness in the construction is that the creator gets to skip this work.