Doxygen - Generate documentation from source code

ManualsLib - Just enter the keywords in the search field and find what you are looking for!

Slate API Docs Generator - Create beautiful, intelligent, responsive API documentation.

Dozuki - Dozuki is a web-based tool for creating and distributing step-by-step documentation.

LDoc - A Lua documentation generator

Manuzoid - Repository that includes user manuals for various products