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" />
  </outline>
</outline>

Update at 10:03am: Forgot to post the executable version (.NET framework required) and the code.