Doxygen - Generate documentation from source code
ManualsLib - Just enter the keywords in the search field and find what you are looking for!
DocFX - A documentation generation tool for API reference and Markdown files!
Dozuki - Dozuki is a web-based tool for creating and distributing step-by-step documentation.
NDoc - NDoc generates class library documentation from .
Manuzoid - Repository that includes user manuals for various products