Two tools that I love enormously are Google Calendar and my iPhone. Google Calendar is incredibly powerful. It allows you to have multiple calendars with any mix of private and public/shared calendars you like. It has a powerful quick ad option for adding events with plain English, amazingly configurable repeating events and reminders, and the feature list goes on and on.
When Google launched Google Sync earlier this year the loop from my Google Calendar to my iPhone and back was completed. Sync allows you to view and edit events on either your iPhone built in calendar or on Google Calendar on the web and see the changes propagated almost immediately.
Google sync used to only allow 5 of your Google Calendars to sync to your mobile device but I noticed yesterday that they now allow up to 25 calendars to sync. If you are a Google Calendar power user this is music to your ears.
I highly recommend this combination of tools. If you haven’t tried it yet you are missing out on some serious awesomeness.