RTF

AscToRTF
A commercial text to RTF converter that recognizes headings, bullets, tables, and URLs.
http://www.jafsoft.com/asctortf/

Rich Text Format 1.0 Specification
Original Microsoft specification, in plain text format.
http://latex2rtf.sourceforge.net/RTF-Spec-1.0.txt

Rich Text Format 1.6 Specification
Technical specification detailing the language and syntax of the format. From the MSDN library.
http://msdn2.microsoft.com/en-us/library/aa140277(office.10).aspx

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.
http://www.rtf-to-xml.com/

Wikipedia: Rich Text Format
Encyclopedia article on the format.
http://en.wikipedia.org/wiki/Rich_Text_Format