MindMeister,
http://www.mindmeister.com is a collaborative mind
mapping application that includes an API. I've tested a bunch of these
types of apps and I've found MindMeister to have the richest feature
set and the slickest GUI. I'm also a big fan of Evernote and would
love to be able to share information between these two applications.
Evernote is great for storing random information, whereas
MindMeister's strength is in creating order from chaos. However,
there's a lot of overlap and there are many items that I would like to
be able to access from either application. Therefore, I think it would
be great to have a MM connector that would allow you to publish items
to Evernote and vice versa.