Open Workbench is a desktop tool that runs under Windows; more information is at http://www.openworkbench.org/. Open Workbench is open source and free.
Openproj is a desktop tool that will run under any operating system that supports Java; it is available at http://openproj.org/. It is also open source and free.
dotProject is a web-based project management application, designed to provide project layout and control functions; it is open source and free. More information is available at http://dotproject.net/.
Another web-based project management application is http://www.gantter.com/; it is free. It is especially good at making Gantt charts.
WBS Tool, available at http://www.wbstool.com/, is a free web software for building Project Work Breakdown Structures (WBS), WBS Charts, Organograms and other types of hierarchies.