If you are a Google Apps junkie like I am, chances are that your iPhone (or WinMo) experience has been lacking. Luckily, Google has released Sync support to instantly coordinate your Google Calendar and Contacts.
All of this was possible before through a smartphone's web browser (and Google Sync has been available on Blackberries for months), but if you were looking to push update your phone to various Google-stored information through the iPhone or Windows Mobile, it was pretty much a read-only experience. Now you can treat Google's calendar and contacts like any push service, updating your phone to the cloud in real time.
UPDATE: I'd mentioned that Gmail was included. It is not.