You can check out https://kanbanflow.com
It's free for now because it's in beta and they say there is no time limit. It behaves very similar to AgileZen
I second the google doc, or you could use an online collaborative board that multiple people can edit.
Or you can host a more robust excel doc in skydrive from MS. I haven't tried that yet.
Mura.ly is another one that I am playing with currently. It has unlimited collaborators, though I think you would probably have to invite them everytime?? with a free account.
Hope that helps!
