Exchange Server 2010 SP1 Beta incorporates a number of feature updates including archiving and discovery enhancements, a faster Outlook Web App (OWA), upgraded mobility features, and several improvements in the management UI. The SP1 beta is available to the public and can be downloaded here.Microsoft-Exchange-Server-2010

Note:  this downloadable software is for evaluation purposes only and is not a released product. If you plan to install the software on your primary computer, it is recommended that you back up your existing data prior to installation. Before you install the Exchange Server 2010 SP1 Beta, we recommend that you review the summary of system requirements and technical information located in the Exchange Server 2010 Technical Documentation Library.

This time-limited, free pre-release version of Exchange Server 2010 SP1 will end 120 days after installation.
To learn more about Exchange Server 2010, visit http://www.microsoft.com/exchange/2010.