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The XML FAQ — Frequently-Asked Questions about the Extensible Markup Language
This is both a plaintext formatting syntax, and a Perl script that converts it to XHTML. The objective was that the souce file should not look like markup (so it could be read just as a plaintext document), but that it should be sufficiently robust to withstand conversion.
Details are at http://daringfireball.net/projects/markdown/.
There are several of these: the best-known is probably the MediaWiki format used by Wikipedia, which provides fairly extensive plaintext facilities for creating the entries used in the site, which are then converted to HTML for display.
Similar conceptual formats are used by some other wiki systems such as PmWiki and Google Code; and there are hybrids which also use HTML like Fossil.
The LATEX document-preparation and typesetting system (Lamport, 1994) uses macros built on TEX syntax to implement a partially-structured document model. The syntax uses the backslash to flag markup elements and formatting instructions, and curly braces to delimit arguments and restrict the scope of typographic actions.
The software is available on all platforms, and creates PDF or PostScript output. Conversion into LATEX from XML-based systems can be done with XSLT, but conversion out of LATEX syntax is problematic because it has features not easily representable in other systems.
SGML and some of its progenitors remain available but are rarely used.
There is an extensive list of other markup languages in the Wikipedia page. JSON is not a declarative markup language.
