documentr is a Web-based tool for editing and presenting software documentation. It allows you to easily maintain documentation for multiple products and product branches. Edits can easily be copied between branches, with merge conflicts being handled gracefully. It uses Markdown as its markup language, along with some extensions, and has a role-based permission system.
The wiki has become the clear choice for use among projects for their help documentation. Wiki Web Help takes the wiki concept and tailors it for use specifically with help documentation. It combines the best of both worlds, with operation similar to a chm viewer and the Web technologies that enable community involvement. It features a dual split pane with tabs for searching, index browsing, and display of contents (in a tree). Users can create and modify topics and pages. Pages can be tagged for index creation. Searching with highlighting is included.