Exit Strategy [1] is a collaborative story where the main fiction has been written by Douglas Rushkoff, but footnotes are added by people going to the website. The story itself is set a few years in the future, but the footnotes are to be written as if it's 200 years further on and a bunch of archeologists and historians are annotating the book.
I'll skip the ethics of Douglas Rushkoff, who is expected to take the best footnotes and publish the result, giving the contributors a signed copy of the book (not all of which he wrote), but I find it neat that any portion of the book can be marked for connotation. You can even add footnotes to the footnotes and possibly even engage on a conversation with fellow anthropologists/historians.
[1] http://www.yil.com/features/feature.asp?volume=rushkoff&issue=01&keyword=rushkoff