OpenProj

OpenProj is a desktop replacement for Microsoft Project.
It is capable of sharing files with Microsoft Project and has very similar functionality (Gantt, PERT diagram, histogram, charts, reports, detailed usage), as well as tree views which aren't in MS Project.

Positive

•  Can import and export MS Project files.
• Has lots of charts types (Gantt, PERTT, Network, Resource usage...)
  

Negative

• Slow because its a pure Java program.
• Not included in the Ubuntu repositories, have to be installed from Debian package (the program checks if there are updates available by itself).

Gantt Project

GanttProject is a free and easy to use Gantt chart based project scheduling and management tool.
Its major features include:

• Task hierarchy and dependencies
• PDF and HTML reports
• WebDAV based groupwork

Positive

• MS Project import/export
• Gantt chart, Resource load chart, Generation of PERT chart, but not so many as OpenProj.
  
• Multi-platform, so it can be used on Windows also.
  

Negative

• Complicated to install on Ubuntu, 'cause there's no Debian Package, nor is it located in the repositories. Use the following instructions for installation:
  wget http://merlinux.org/ganttproject/ganttproject-2.0.6-1mer.noarch.rpm
  sudo aptitude -y install alien
  sudo alien -i -d -c ganttproject-2.0.6-1mer.noarch.rpm
  This will install the application in /opt and add an entry in the Applications menu.
  Unfortunatly, in my system, I'm not able to run the application as normal user, somehow I miss the $TMPDIR variable which is used for logging errors.
• Pure java application, therefore its slow and the gui doesn't fit into the desktops look.
  

Planner

Planner is a Project Management application that supports Gantt charts, resource allocation and other basic stuff you would expect from such kind of application.

Positive

• Fast, smooth and easy to use.
• Good looking aspect.
• Well formatted printing and HTML output.
• Can be easily converted to Timeline format.
• Located in the Ubuntu repositories.
  

Negative

• Only supports its own file format.
• Only Gantt chart type support and resource usage (no PERTT, ...)
  

opensched

This program is used to automatically schedule resources for a project.
It takes as input a file describing the project and generates the following:

• Textural descriptions of the generated project plan.
• Gantt charts.
• Network diagrams.

The textural description can be generated in one or more of raw text, HTML, and TeX formats.
The Gantt charts and network diagrams are generated directly as EPS drawings.
The package contains a routine to convert these to GIFs.

Negative

• No graphical user interface, so its very hard to use (maybe useful for batch and automated processing).
• The generated graphics are very simple and ugly.