Read the Docs hosts documentation for the open source community. It supports Sphinx docs written with reStructuredText, and can pull from your Subversion, Bazaar, Git, and Mercurial repositories. The code is open source, and available on github.
The documentation for the site is organized into two different sections below. One is for users of readthedocs.org, that is the first section. The next section is for users of the code that powers the site. All of the RTD code is open source, so you can run your own instance.