1- Assign A Day
Assign-A-Day is a free tool designed to enhance teacher and student communication through an online teacher-managed calendar. Teachers create a calendar for each of their classes and add assignments for the students to view. Students view their teachers' calendars in order to see assignments for classes they might have missed, or to get an overview of the class.
2- Google Calendar
Google Calendar sets the standard for drag-and-drop editing inside of its uncluttered interface. Armed with more than just good looks, rights-assignable group calendaring and integration with GMail's e-mail and contacts databases make it a good choice for the workplace.
3- Short Calendar
This is one the simplest calendar tools out there on the net. Short Calendar lets you organize your tasks into a neat format that you can share with others via email.