As a matter of course here at OStatic, we're committed to compiling documentation and guidance resources for popular open source platforms and applications. After all, one of the most common critcisms of open source creations is the lack of official project documentation. One of the best ongoing projects for producing free open source-related documentation is FLOSS Manuals. It's an ongoing and ambitious effort to build online guides for open source software. Recently, the site has added useful documentation for some projects that may interest you, including Firefox and the interesting collaborative editor Etherpad.