Microsoft announced today the availability of a release candidate for their next server OS, based on Vista. Windows Server 2008 has now been released as Release Candidate 0, which can be downloaded for review.
You'll need a TechNet subscription to download it on your own or have an MS partnership, and obviously as this is an RC it is not for use on production servers. However, as it is an RC it is an indication that they are nearing a finished product, and are looking for final feedback before it goes retail. They are making good on their promise last week to make a test version available, so if you are eager and willing go give it a shot.