Markdown is a WikiSyntax that resembles plain text e-mail formatting.  It is the default markup used in IkiWiki. Also supported by InstikiWiki.

See http://daringfireball.net/projects/markdown/syntax

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Perhaps more famously, Reddit uses MarkDown.

--PaulMiller
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There is a tool called PanDoc which can convert between MarkDown and ReStructuredText and a number of other TextFilter formats.
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CategoryTextFilter