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...
Dr.Explain - Dr.Explain software is a help file authoring tool.
ExtraCHM - A fully featured reader for Compiled HTML (chm) documents.
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.
Microsoft HTML Help - Microsoft HTML Help is the standard help system for the Windows platform.