I’ve been helping out Phillip and Matt the past few days with xNote, an open source C# OPML browser. Here’s a screenshot of what we have so far:
I just recently added:
saving the OPML document to disk
editing text attributes
editing comments
So now it’s less a browser and more like an editor. You can only edit the top-most document, not documents loaded by inclusion links (blue folders).

Still on the TODO list:
edit link/inclusion URL (modal dialog?)
add new nodes
delete nodes
from the spec: “By convention if an outline is commented, all subordinate outlines are considered to be commented as well”
Here’s a question that wasn’t answered by the spec: can links have comments? In the implementation of xNote so far, they can. Here’s what that looks like in OPML markup:

<outline text=”This is a demo directory”>
<outline text=”This directory …” type=”link” url=”http://www.opml.org”>
<outline isComment=”true” text=”make a comment on a link” />