A standard format for storing and rendering outlines. The file format is XML formatted text.

Outlines are the foundation of many file types including project documents, presentations, mind maps, directories, indexes and note taking applications. Via OPML, applications of these types can exchange information without loosing their nested hierarchal structures.

Although there are OPML editors for Windows, more commercial OPML aware productivity applications today are written for the Macintosh. The Mac ships with the excellent OmniOutliner product. On a Mac it is possible to exchange outline data between OmniOutliner, project management tools (Merlin, OmniPlan), graphics (OmniGraffle), presentation tools (Keynote), MindMapping tools (NovaMind, MindManager also avail for Windows) and other tools.

Related Links
Download a free OPML Editor (all platforms)
Wikipedia on Outliners