Google Apps Sync

From Google:

Google Apps Sync for Microsoft Outlook is a plug-in for Outlook 2003 or 2007 that lets you keep using your familiar Outlook interface after switching from Microsoft Exchange Server to Google Apps. Use nearly all the Outlook features you're used to with Exchange to manage your Google Apps Mail, Calendar, and Contacts.

Google Apps Sync is designed to help sync all your email messages, calendar events, and contacts in Outlook seamlessly with your Google Apps account in the cloud, allowing you to access the same information at any time, from either interface. Importing your existing data from Exchange is easy, too, so you'll waste no time in making the transition.