Google Calendar
Google Calendar is much like Google Docs in that it allows multiple users to view and potentially edit the document. Online calendars allow teachers to edit and add to the calendar throughout the month and year, unlike hard copy calendars which cannot be changed once they have been distributed. Settings can be changed so that only the administrator of the calendar can edit and add events, or can be set up so that parents and others can add to the calendar.
Events can also be managed by type. For instance, classroom activities can be one color (green), while school wide events can be another (blue), birthdays another(orange), and holidays even another color(dark green)! All of this is secure, like in Google Docs, so only those invited to the calendar can view.