For most of the history of technical communication, docs have been a megaphone that enable one-way declarations from tech writers to product users. In many cases, tech writers still perceive docs in that way.
However, user behavior has diverged from this model. Since web search became good and readily available, product users no longer exclusively rely on the official docs. Now they listen to whatever the search engine deems to be the most reliable answer to their question. Sometimes, this comes from the official docs. More often than not, though, the most useful information comes from forums, blogs, YouTube, and more.
As tech writers, we're at a crossroads. We can either ignore the fact that the world has changed, or we can embrace it and realize that docs are now a multi-way conversation, of which tech writers only have a single voice. Our job is still to make the most of that voice, but it's also to facilitate the rest of the conversations and make them more accessible to users.
Two examples of sites that do a good job of this are:
Contributor profiles are a major focus of both sites, and the contributions of each user combine to create a site-wide reputation for that user. iFixit has a particularly good algorithm for establishing and showcasing contributors' reputations.
For both sites, the interface for contributing content is quite easy to use, with WYSIWYG authoring environments and brief but useful inline guidance. The Instructables site has excellent guidance on how to write good content (although it would be nice if this page were easier to find.)
What are other important considerations for collaborative sites? What are examples of your favorite sites?