Project management tools

Free

In this 2016 article at Opensource.com the top 11 project management tools for 2016 that are Free / Libre Open Source Software (FLOSS) , they list

1. MyCollab AGPLv3, Java, MySQL
2. Odoo GPLv3, limited feature community edition
3. Taiga GNU Affero GPLv3, stack includes Nginx, Python and PostgreSQL You can start a project at taiga.io
4. OrangeScrum GPLv3, based on CakePHP
5. Tuleap GPLv2 - a large complete system
6. Agilefant (license)
7. Redmine GPLv2 RoR
8. [Project-open] (dual license)
9. OpenProject GPLv3
10. LibrePlan GPLv3 Java, PostGreSQL, Tomcat
11. ProjectLibre Common Public Attribution License v1 Java

I personally have used and would recommend TaskJuggler and FreeMind as well as simply organizing content and project documentation in a wiki.

Non-free

If you use Google Apps, or if you just want a quick online tool, you might want to check out http://gantter.com/gantter-editions/

Interesting and easy to use is Kanboard MIT license

Free / Libre Open Source Software = Another attempt to de-emphasize the price connotation of 'free' and instead recognize that free means liberty.

Gnu's Not Unix, because Unix is proprietary software while Gnu is software that respects your freedom.