RTF

AscToRTF - A commercial text to RTF converter that recognizes headings, bullets, tables, and URLs.

Rich Text Format 1.0 Specification - Original Microsoft specification, in plain text format.

Rich Text Format 1.6 Specification - Technical specification detailing the language and syntax of the format. From the MSDN library.

RTF to XML - Converts RTF documents to either XML documents or XSL/XML document sets that are compliant with the XML FO specification. System requirements, sample conversions, and documentation. Commercial software, with free evaluation version.

Wikipedia: Rich Text Format - Encyclopedia article on the format.