ManualsLib - Just enter the keywords in the search field and find what you are looking for!
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Internet Archive Manual Library - Internet Archive Manual Library is your go-to source for viewing the big collection of datasheets, instructions, and guides written for a wide range of products.
Dr.Explain - Dr.Explain software is a help file authoring tool.
Manuzoid - Repository that includes user manuals for various products
CHM Editor - Best solution to edit CHM files