Scrum is an approach to Project Management that is very different from the traditional models such as waterfall, and is an agile development method. Scrum involves integrating a cross-functional team into one cohesive team that delivers software as per defined intervals known as Sprints. The set of features that are used to define a Sprint are derived from a product ‘backlog’, in a meeting where the Product Owner states the features that he/she wants completed in the Sprint. An integral part of this approach is that the team understands that feature definition has a very high chance of changing during the cycle.
A tool used for Scrum should be able to demonstrate this required flexibility. One of the tools that can be used is called ‘Scrumy’ (link to site).
Some reviews of Scrumy:
http://borisgloger.com/2008/11/15/scrum-tools-scrumy-pro-review-update/
http://www.jesseharding.com/blog/2008/09/29/scrum-project-management-with-scrumycom/
Have you tried http://www.scrumedge.com? I like it much better than scrummy