I still recommend that many businesses check out Google Applications for their email system. The standard edition (which will meet the needs of almost all businesses) is available free for one hundred users.
Google just announced the ability for Outlook to sync with the Google Apps servers. Unfortunately this is only available to users of the premier edition. So you will have to fork out $50 per user/per year, but for those who can't get away from Outlook, this might be a good alternative to paying for Microsoft Exchange.
Google Outlook Sync has the following features:
- Fast mail sync between Outlook and Google.
- Full calendar sync, including recurrence patterns and attendees.
- Contacts sync for all Outlook fields.
- A Global address list with email auto-complete contacts lookup.
Read more about using Google Apps with Outlook.