Pivotal Tracker is a web-based agile project management tool that is used almost exclusively for Scrum development. It has points built into stories and provides burnup, burndown, and velocity charts out-of-the-box. It allows developers to collaborate in real time around a shared, prioritized backlog, with features that include automatic planning, multi-project workspaces and performance analytics. It can also be used for issue tracking.
Pivotal Tracker is inflexible in the sense that it is not easy to customize, but it has a large pool of integrations that extend its functionality. It's best suited for big development teams with well-established processes.