Windows 2003 R2
Windows Server 2003 R2 is available for download as a 180 day trial. The Update to Windows Server 2003 SP1 offers increased capabilities in branch office deployments, Identity and Access Management, a new and improved Distributed File System, and the anxiously awaited Active Directory Federation Services. I would recommend Using the associated reviewers guide to enhance the trial experience and to help identify the functions of the new features.
I love this product. My personal favorite features include the Active Directory Federation Services (ADFS) which allow for extranet access without the associated problems inherent in building a forest to forest trust relationship. There are so many great uses for this service. I also loved the synchronization updates to DFS. Take a few minutes to download and start working with Windows Server 2003 R2.