OPML an XML-based format that allows exchange of outline-structured information between applications running on different operating systems and environments.
In the summer of 2005 Dave Winer, released an open source OPML Editor, for Windows and Mac OS.
To learn more about outliners, the software category that inspired OPML, check out outliners.com.
The OPML spec has a liberal copyright, adapted from the copyright of the IETF, which has stood up well over time.
Dave Winer's OPML Validator came online in October 2005.