texinfo

Texinfo is a documentation system that can produce both online information and printed output from a single source file. Normally, you'd have to write two separate documents: one for online help or other online information and the other for a typeset manual or other printed work. Using Texinfo, you only need to write one source document. Then when the work needs revision, you only have to revise one source document. The GNU Project uses the Texinfo file format for most of its documentation.