What are the opensource alternatives for MS Project in order to manage Project management?

GanttProject Open Source by Dmitry Barashev, Alexandre Thomas | Mac, Windows, Linux

GanttProject is a cross-platform desktop tool for project scheduling and management. It runs on Windows, Linux and MacOSX, it is free and its code is opensource.

ProjectLibre Open Source | Mac, Windows, Linux

ProjectLibre is a Java-based project management solution that gives Microsoft Project a run for its money. Offering a UI similar enough to ease the transition from Microsoft's market leader.

GNOME Planner Open Source by Richard Hult and Mikael Hallendal, | Windows, Linux

Planner is the GNOME project management tool. Its goal is to be an easy-to-use no-nonsense cross-platform project management application.

Calligra Plan Open Source by Calligra Suite | Linux

Calligra Plan (formerly KPlato) is a project management application. It is intended for managing moderately large projects with multiple resources.

2-Plan Free | Mac, Windows, Linux

2-plan Project Management Systems offers three PM tools: a free desktop system, an open-source software for multiple projects and teams, and a scrum board.

Source: http://alternativeto.net/

You can also try RationalPlan Single. It is not open source but it is free for Linux users.