Can I use a different email/calendar/address book provider for Mac OS X and iOS?
Googles Services

iOS


(Push) Email using the IMAP protocol


Push is not default. You have to set it up manually.

bi-directional contacts sync (Exchange supported)


If you do not sync contacts using the exchange protocol, you only sync via iTunes.

bi-directional calendars sync


I don't have good experiences with the bi-directional sync using the iOS calendar app. Luckily, there are plenty of third-party apps available which offer a tight integration of Google Calender. CCal has been quite an enhancement to my Google experience.



OSX

iCal, Address Book and Mail allow to integrate sync with Google's services in Preferences &#x2192; Accounts.

Google Services on other platforms

Google's Services are more widely available than anything else I know. It allows to keep contacts/calendars/email in sync on OSX, iOS, Android and Linux. In Ubuntu I use Google's Services in Thunderbird and Google Calendar/Contacts extensions.