Want to try out Windows Server 2008? Or perhaps Exchange Server 2007? You can download complete virtual machine environments for a whole list of Microsoft products, free of charge. Check out the Test Drive program here: http://technet.microsoft.com/en-us/bb738372.aspx for more information and the download links.
The best thing about it is that you can download an entire virtual machine with the software pre-configured and running correctly – so you don’t need to complete any additional tasks in order to start playing/testing/evaluating the products immediately in your own virtual environment.