The Zimbra Collaboration Suite (ZCS) stands out with tight integration of e-mail, calendar, contacts, IM, and even XML-based documents and spreadsheets. The docs have been left behind, but the mail, calendar, and contacts are fully supported on iPhone via the Zimbra Web Client (right). Even better, the spanking new Zimbra for iPhone 2.0 (left) supports direct, automatic synch from ZCS to the native Apple iPhone mail, address book, and calendar apps.

Zimbra