I would like to share one iPhone App for Project Management and To-do list which called Producteev.
It is task management tool with cloud technology so that you can sync your task everywhere. Furthermore, it has iPhone, Android , Mac and Windows version so that you can sync your task in every platform. The free version can invite one more person into your "workspace" which is a "category" in the application holding all the tasks within the same project. Once you invited people into your "workspace", you can assign tasks to him/her and they will have the task in their workspace.
Each task can have discussion part so that you can update or sharing your thought in the particular task.
It is useful and powerful than a general to-do list tool.
And share here :-)