Google Calendar is one of Google's many free resources that I use. It has some features that make it very useful.
It's accessible on any web browser and integrates and syncs with smartphones, Outlook, Apple iCal and Mozilla Sunbird.
I have all of my appointments, due dates, deadlines, etc. on it. I color code them based on school, blog, degree program, meetings, fun, etc. My wife and I share our Google Calendar's with each other so we can easily see each other's schedules. If you use Google Tasks and set a due date for the task, it will show up on your calendar for that day. I also have US holidays and the Yankees schedule on my calendar.
The reminders feature makes sure you don't miss an appointment or forget to do something. You can set it to just have a pop-up reminder (which also triggers an alert on your Android Phone), send an email, or send a text message and set a time for the reminder.
You can invite people to an event you have set up and then they can RSVP via email or Google Calendar.
It also makes it easy to set up appointments. This is great for administrators or educators who want to make it easier to schedule appointments with staff, parents, or students. The Appointment Slots feature lets you set time slots on your calendar that other people can sigh up for. This is great for office hours and parent conferences.
You can also share your calendar on blogs and websites. Create a calendar for your school or classroom and post it on your site with upcoming events.
Google Calendar is very powerful, easy to use, very useful, and free.