AsciiDoc is a plain text markup language for writing technical content. It’s packed with semantic elements and equipped with features to modularize and reuse content. AsciiDoc content can be composed using a text editor, managed in a version control system, and published to multiple output formats.
The Darwin Information Typing Architecture (DITA) specification defines a set of document types for authoring and organizing topic-oriented information, as well as a set of mechanisms for combining, extending, and constraining document types.
DocBook is a schema (available in several languages including RELAX NG, SGML and XML DTDs). It is particularly well suited to books and papers about computer hardware and software (though it is by no means limited to these applications).