Agile software development relies heavily on a work item type called user stories. For example, you have a backlog full of user stories and you can select a few of them to work on during the next sprint.

But where and how do you find user stories to put into the backlog? There is a popular technique for doing that called story mapping. Jeff Patton invented it and here is the definitive guide on how to do it.

The question is, what electronic tools are out there that support Patton's story-mapping technique?

I've done a bit of research, found Pivotal and Rally plug-ins (but I'm not a customer of either) and I'm currently experimenting with SilverStories.

What other tools are out there? What have you used? What do you (not) recommend? Why?

UPDATE: Some people who wrote comments seem to lean towards an answer that applying this technique is simply impossible with an electronic tool and we should just accept that. Can't someone write it up as an answer?