HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Kchmviewer - Kchmviewer is a free, open-source CHM (MS HTML help) and EPUB viewer for Unix, Mac and Windows...
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
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MadCap Flare - Documentation for Any Audience, Language or Format