Let's start with OpenProj. OpenProj doesn't cost anything and it is open source, meaning you can download the source code and modify or extend the software, if you know how to program. It runs on Linux, Microsoft Windows, and Mac operating systems. It provides a complete replacement of Microsoft Project and can open Microsoft Project files.
OpenProj uses an advanced scheduling engine, provides Gantt charts, network diagrams, WBS charts, RBS charts, and more. Translations exist for Spanish, German, French, Chinese, Russian, Portugese, and Swedish.
Download files are available here. Ubuntu users download the debian package (.deb) and follow the instructions for quick and easy installation.
TaskJuggler is another free project manager for Linux. It has various features that go beyond commonly used Gantt chart editing software. It provides support during all states of project management, starting with the first concept and ending with project completion.
Besides implementing all the standard features of typical management tools, such as time lines, resource management, and contact lists, TaskJuggler includes a so-called Resource Conflict Solver, and allows you to assign running costs to resources.
Data can be exported in stardard CSV and iCal file formats for exchange with office software and productivity tools. Furthermore, it provides project tracking for clients, and supports multiple time zone planning.
Project Open combines project management with enterprise resource planning (ERP) functions. It is one of the largest open source projects and used by some 1000 companies around the world. Features include task planning using GanttProject, resource assignment and tracking, budget setting, and invoicing for fixed price and effort based projects. Download information for RedHat/CentOS, Gentoo, Debian, Ubuntu, and Novell/SuSE Linux distributions can be found here.
ValleySpeak is neither free nor open source, but it can provide some cost savings when multiple team members need to access a project file from various locations. ValleySpeak enables you to make a Microsoft Project file available from the Internet, so that any number of participants can access it through a web browser. This means considerable savings on licenses for Microsoft Project and there is no requirement for additional software installations.