ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Docsie - Docsie makes it easy to create and maintain your website documentation, without having to deploy it...
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
ReadMe - A collaborative developer hub for your API or code.
Dokit - The solution for disseminating know-how: work instructions, procedures, knowledge base, documentation techniques, product support, wiki enterprise.