OPML
by Amit
OPML (Outline Processor Markup Language) is an XML format for outlines. Originally developed by Radio UserLand as a native file format for an outliner application, it has since been adopted for other uses, the most common being to exchange lists of web feeds between web feed aggregators.
The OPML specification defines an outline as a hierarchical, ordered list of arbitrary elements. The specification is fairly open which makes it suitable for many types of list data.
Programming and writing about it. 