Dave Winer mentioned two programs other than Frontier/Radio that support OPML. One is a Mac program, but the othe is an outliner written in Java called JOE, which I downloaded and tried out. It’s a nice Radio-style outliner, which reads/writes OPML & plain text, and has a pretty (but sluggish) interface.
My problem with both JOE and Frontier (which it resembles) is that it ‘s all rather autistic – what’s the output of the outline? I still prefer the outliner in Word for organizing thoughts, because it produces documents, and it has styles, and shortcuts, and it’s a decent writing tool. And for programming, you really want an editor that handles all the languages I code in, like Scintilla. Outliners are perfect matches for Usertalk, Python, and that’s it, basically.