I've been an avid user and recommender of Pivotal Tracker, an excellent, simple to use, Scrum based project manager. I still love it a lot.
But, today I came across a very clean and simple alternative called Trajectory.
You may want to check it out. I can't say why we need Trajectory where we have Tracker. My guess is that Trajectory is much less than a scrum system, maybe it's a structured way to communicate stories and design between client and developer.