We just launched our Online hosted offering for enterprise customers.  This offering lets our customers offer Email, Instant Messaging, Web conferencing, collaborative platform and enterprise portal to their organization.  Its all hosted at the Microsoft data center and a directory integration service is provided to make sure there is single sign-on.

Business Productivity Online Standard Suite
Starting from $15 per user per month
Suite includes: Microsoft Exchange Online, Microsoft SharePoint Online, Microsoft Office Live Meeting, Microsoft Office Communications Online

30 days trial available here https://mocp.microsoftonline.com/site/default.aspx