A lot of the open source projects for Scrum tools have been abandoned recently. In a recent article, the ScrumExpert web site has published an article listing the commercial scrum tools that you can use for free, either after downloading them or on a hosted base.
In this list you will find a lot of the well known name in the Scrum tools industry like IBM Rational, Rally or VersionOne. You will also find smaller companies like tinyPM or the hosted version of open source projects like ScrumDo or IceScrum.
You can find this complete list of commercial scrum tools that you can use for free on http://www.scrumexpert.com/tools/using-commercial-scrum-tools-for-free/