GanttProject seems to be a good replacement for Microsoft Project. It is in active development, and it's supposed to run on different platforms since it is written 100% in Java. There is a bug that shows up in both Jaguar and Panther that prevents any of the text and icons in the window and its menu from displaying. The bugfix is to replace a line in the preference file. However, it doesn't describe how to do it.
Read on if you would like to take another step to free yourself from Microsoft.
The problem is that the preference file where you have to change the line doesn't exist until you quit the program. So in order for the program to create the file, you have to fumble through the window's messed up menu to find the Quit menu item. (Do not quit from the menu at the top of the screen; you have to do it from within the window.) Afterwards, you can follow the directions in the bugfix to change a line in the .ganttproject file in your home directory from (shown as two lines for space reasons):
<looknfeel name="Kunststoff"
class="com.incors.plaf.kunststoff.KunststoffLookAndFeel"/>
to
<looknfeel name="Mac OS X" class="apple.laf.AquaLookAndFeel"/>
Alternatively, you can create a new .ganttproject file in your home directory with the following text. This assumes that you are using version 1.9.10, and you put the .jar file in your main Applications directory. (You can then just doubleclick on the .jar file to run the program.)
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ganttproject.sourceforge.net>
<ganttproject-options>
<language selection="en"/>
<task-color red="140" green="182" blue="206"/>
<geometry x="315" y="157" width="810" height="608"/>
<looknfeel name="Mac OS X" class="apple.laf.AquaLookAndFeel"/>
<automatic-launch value="false"/>
<dragTime value="true"/>
<tips-on-startup value="true"/>
<redline value="true"/>
<xsl-dir dir="jar:file:/Applications/ganttproject-1.9.10.
jar!/xslt"/>
<xsl-fo file="jar:file:/Applications/ganttproject-1.9.10.jar!/xslfo/
ganttproject.xsl"/>
<working-dir dir="/Users/louie"/>
<files>
</files>
</ganttproject-options>
IMPORTANT NOTE: The xsl-dir and xsl-fo lines above are split for a narrower story display. When entering the code, remove the line break but do not add a space!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040111141314585