I've started working on my Bayesian RSS classifier again (I'm tinkering with both rss2mail and a PHP proof-of-concept), but being a stickler for proper UIs, I'm starting with the OPML editor.
(By the way, Peter Bowyer led me to what seems to be the first Bayesian RSS classifier with an UI. Neat.)
As usual, I have a pressing itch to scratch - I'm fed up with having all my current readers slightly out of sync with each other, and need some way to maintain a central OPML file.
In the meantime, I'm jotting down some useful techniques: