What is the best, open source Scrum tool available today?

If you are looking for a good Open Source Scrum tool, have a look at XPlanner Plus. May be it's currently not the best one, because it lacks some modern features:

• releases or roadmap.
• task board/dashboard view.
• story points.
• ajax based usability features.

still it's good enough. XPlanner+ is web based and has fancy design. XPlanner’s UI is straightforward and intuitive. Xplanner has built-in reports and charts.

As now, XPlanner+ is in the process of intensive development, I know XPlanner's weaknesses will be fixed.