I am currently using the community edition of RallyDev and am very happy with the results.
It is geared towards Agile project management, and is easy, intuitive and mostly full featured. The only difference between this and the paid version is that you are able to handle multiple projects per account, and you get a few more special reports. It is pretty easy to work around however by simply having multiple projects as different "Releases" and sharing a backlog of user stories between them. I simply tag the user stories with the correct project and that allows me to filter on them.