Markdown is a text-to-HTML conversion tool for web writers. Markdown info is available at Daring Fireball.
The Smartypants and the Markdown+SmartyPants perl scripts add additional support for Markdown to HTML conversion. (See documentation at Rentzsch Contemporary Quill.)
The Ruby language has support for Markdown in the Bluecloth project. Bluecloth is available through RubyGems.