wordpress plugin or standalone tool needed for task management

I was searching for a tool(wordpress plugin preferred) which can manage my task list.

requirements are simple as follows :

- global task list for members
- task status pending/completed etc.
- member can apply/self-assign a task and admin will accept/assign***
- member can suggest a task, admin will accept and add to global task list
- comment feature under a task

***must be needed this feature

Can anyone point me a free/opensource tool/plugin? Thanks!

via Chebli Mohamed