iCloud - Apple's best kept secret
It still surprises me when I meet an iPhone or iPad owner who doesn't know about iCloud.
iCloud is a free service Apple provides that enables you to:
1. Back up your iDevice
2. Find your iDevice if/when you lose it (and remotely erase it, if you think it has been stolen)
3. Synchronise your Address Book contacts, Calendars and email across all your iDevices, Macintosh and Windows equipment
4. Access all the information in (3) above using a web browser on any internet connected computer in the world.
(Did I mention its free?)