Gantt Project

Here is a fantastic open source project organizer similar to Microsoft Project. It is cross platform so you can use it on mac (X only L) and linux as well.

http://ganttproject.sourceforge.net/

I have actually known about this for a while, but I saw it in action on the MakeHuman (which is also worth taking a look at) site.

There are also lots of third party tools, which I haven’t played with, so I can’t really say much about them.