Collaborative Creation

These collaboration systems let participants contribute to an aspect of a creation, typically artistic.

Interactive Genetic Art I and II and Movies allow readers to pick favorite mutants.
Genetically Programmed Music does the same for audio.
The Morph Lab page lets you interact, while others are too, to select the next generation of Morphs, and save them to a collection.
HypArt - The thought is that a single picture is created by several people. Therefore the image is being divided into squares and each artist contributes one of them.
The OTIS SYNERGY: PANIC GRID project is similar. - A grid is the collaborative effort of several image-makers created using a grid of squares which the artists fill with images that interconnect and play off one-another.
Collaborative Visual Art - 5 x 5 Project
WebWorld is a virtual world you can move around in, build in, and visually link to other parts of the World Wide Web.
PING map - See the objects in the ping landscape by clicking their icons on the map below, or place your own object at an empty spot.
Stories from Downtown Anywhere are added to by readers, but not automatically yet.
The IMRF Choose Your Own Adventure Story lets you add to the story if you get their software.
Addventure! lets participants add new rooms to the ever growing adventure world.
Hypertext Hotel MOO and WAXweb MOO are intended to support groups of writers and scholars in writing and publishing hypertext fiction and criticism.
The Caption Competition lets you suggest a caption for a picture, and vote on others suggestions. I've seen another one of these, but I forget where.
Whimsy an experimental multiplayer game, which allows all users to add material, comment on existing material, and pass messages.
Breaking Out (of the Virtual Closet) has the world's first (and probably longest) collaborative sentence.
Tree fiction on the World Wide Web - discusses some of the problems of writing one kind of hypertext fiction, called `tree literature', or `plot branching', or `choose your own fiction' consisting of short sections of conventional narrative, each ending with a choice for the reader that determines what happens next and thus which section of the narrative should be read next.
Genetic Collaborative Design Homepage
Arc, The Interactive Media Festival has several things collaborative.
At No Dead Trees, The Interactive Novel, authors from around the world are exploring hyperexistence via hypertext. We are looking at it via the tree structure.
Collaboratively created list of links to hyper creations, some collaborative