Are there any web-based usable scrum backlog tools which beat excel? I'm especially interested in a product backlog feature, we can do sprint backlog just fine with post-its. The "killer feature" I'm looking for is that I can assign unique priorities for the backlog items. In Excel or text-file this is a no brainer: the order mandates the priority. This is a very basic requirement for scrum backlog to work, yet it seems that the free tools do not support it. Burndown charts would be nice too, but at least it should show the full amount of work left.
I have tried xplanner (didn't have the product backlog feature and seemed dysfunctional otherwise as well) and Agilo (didn't have unique priorities for backlog items).