There have been a lot of ideas about how to allow
multi-writer web pages. The simplest implementation is the classic wiki
(everybody can write everything), the most useless
idea in this area is Annotea
which requires modifications at the client (as proof of
irrelevance, they implemented it for Amaya).
There are many applications where the ability to add
comments would be useful, and where the wiki concept allows
too much mischief.
A group of brazilians implemented what they call
insert links in a text and add links to existing lists of links.
They require no modifications at the browser and the
new links are stored at the server (not always a pro, but a
good start when compared to annotea, where all modifications
are stored at the W3C), but not the content they point at.
A nice application would be, e.g. a distributedly annotated
edition of a literary text.