There is a lot of stuff going on with OPML these days. OPML stands for Outline Processing Markup Language, invented and evangelized by Dave Winer some number of years ago. Since then though, at least from my observation, it's had its greatest traction as an import / export format for RSS Aggregators.
But now, there are lots of cool new ideas floating around, prototypes and products being built, that rely on and leverage OPML. Also there are lots of less new sites which are using OPML in interesting ways.
In short, anyone who uses, generates, processes, filters or renders a hierarchical list of some kind, especially of Feeds, should consider using OPML as input or output or both. That is the way to interop, and lots of amazing leverage. Think of it as a different kind of mashup!
OPML camp can be a gathering where the ones who are 'into' OPML compare notes with each other and discover new directions, and the ones who can't spell OPML get a chance to learn and add their ideas to the mix.
Come to OPML Camp. I'll be there! Here are the coordinates:
When: May 20-21
Where: Harvard Law School, Cambridge, Massachusetts, USA
Signup: click here