Ganttproject [1] is a project management tool written in Java. This enables crossplatform compatibility and eliminates such arguments for not choosing that tool; The project and user information is stored in XML files which offers all the possibilities to transform that data in any other convenient format such as PDF or XHTML. If your werbserver has web_dav enabled, you can directly manipulate project files on the sever and work as a group on the same sources. As far as I can see, a version control is missing and I guess that this could alternatively implemented on the server side with a simple backup or a revision control system.
[1] http://ganttproject.sourceforge.net
