Redmine is a free and open source, web-based project management and issue tracking tool. It allows users to manage multiple projects and associated sub-projects. It features per project wikis and forums, time tracking, and flexible, role-based access control. It includes a calendar and Gantt charts to aid visual representation of projects and their deadlines. Redmine integrates with various version control systems and includes a repository browser and diff viewer.
Redmine is written using the Ruby on Rails framework. It is cross-platform and cross-database and supports 34 languages.
it's flexible, highly customizable, (you can choose between various templates, plugins, etc.) It's open source and free to use. [and] the user interface doesn't overwhelm you.
Since it's highly customizable you really have to dig yourself into it at first in order to create a great experience for your team members and colleagues / clients