Microsoft Exchange Server 2007 Hosted Trial
Like it was the case right after Exchange 2003 RTM, you can also sign up for an Exchange 2007 tempoary account...
"Welcome to the Microsoft Exchange Server 2007 Hosted Trial web site. You can create a temporary account to experience the features of Exchange Server 2007 which will stay active for 5 days. Your account will be pre-populated with sample messages, calendar appointments and a voice message to help you have a rich experience. You will be able to send and receive mail, schedule meetings, and adjust your options. You can access your account via Outlook Web Access, Outlook, or an Exchange Active-Sync compatible application. Please review your pre-populated messages for details."
Sign up for an account here.