reStructuredText
A plain text format that is the defacto standard for descriptive text shipped in distribution files, and Python docstrings. This documentation is authored in ReStructuredText format.
Setext, an old lightweight markup language (1992). Markdown, a widespread lightweight markup language. Textile, a similar lightweight markup language to produce HTML. Asciidoc, a similar lightweight markup language supporting many types of documents. txt2tags, another similar lightweight markup language.