Just used google calendar and noticed the features they are offering, my personal favorites:
1.) Add calendar like contacts b'days, orkut friends b'day, country specific holiday, Cricket Calendar, IPL Calendar.
2.) Notification settings to email, pop-up, SMS (Indian Numbers supported)
3.) Side bar to add, jump to date, check free/busy time by giving gmail address, world clock etc.
4.) Add multiple time zone and they all will be displayed on the left side.
5.) Add location and it will show the weather informtion.