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.
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.