Hi Torsten,

the pivotaltracker is a free, very easy to use and straight forward agile project management tool. We're working with it for nearly one and a half years now and we didn't see anything else that fulfills our needs as good as the pivotaltracker does.

Also check out this link about it: http://www.agileweboperations.com/pivotaltracker-the-simple-steps-to-start-a-new-project/

Greets,
Daniel