Google Calendar and Telecom New Zealand
Google Calendar supports SMS/txt reminders - a pretty handy feature. You can set it to remind you with a txt anywhere from five minutes to two days before an event. Or, you can use it to track responses to invitations you've sent, or changes to events you're attending.
The only problem is that, ostensibly, they only support the Vodafone network in New Zealand. But, luckily, if you're stuck with Telecom, it still works. Just chuck in your normal 027 phone number on the settings window.
It seems to be free, too. Well, I haven't been charged yet.