The PLExD Toolkit | Promethean Learning Experience Design
A collection of free and open source tools for project management, graphic design, collaboration, audio, video, and other utilities.
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Sendtask Features
Free tool to assign tasks via email even if people don't have an account.
Gantter - web-based project scheduling made easy
Web-based project management tool. Can import/export to MS Project. Integrated with Google Drive.
Trello
Free tool for organizing projects. This isn't a standard project management tool, but you could use it to assign and manage tasks in a team.