Redmine
http://www.redmine.org/
Widely used project management tool. Has many plugins.
Redmine Issues List
Redmine Gantt
LibrePlan
http://www.libreplan.com/home/
LibrePlan is a web based project portfolio management system. The community version is free and enables managing projects portfolio, tasks, resources, resource planning etc.
Projects portfolio screen
LibrePlan resources load screen
Taiga
https://github.com/taigaio
Taiga is an open source agile project management tool. Supports both Scrum and Kanban agile methodologies.
Taiga Project Backlog
Tuleap
https://www.tuleap.org/
New open source project management tool, supports agile, integrates with git and svn. Has gantt views.
Tuleap dashboard
Tuleap Kanban
CodevTT
http://codevtt.org/site/
Interesting project that uses MantisBT to manage the tasks. CodevTT provides project management and timetracking of projects. It can alert on delays in projects.
codevTT Projects drift dashboard
codevTT holidays planning
Agilefant
http://www.agilefant.com/product/
http://sourceforge.net/projects/agilefant/
Open source agile based (Scrum and Kanban) project and portfolio management.
Agilefant Story Tree
Agilefant effort
Mantis BT
https://www.mantisbt.org/
Mantis Bug Tracker
Ganib
http://www.ganib.com/open-source/
http://sourceforge.net/projects/ganib/
Supports Scrum agile.