Microsoft has released a trial version of Windows Server 2008

This software is for evaluation and testing purposes. Evaluating Windows Server 2008 software does not require product activation or entering a product key. Any edition of Windows Server 2008 may be installed without activation and evaluated for an initial 60 days.

If you need more time to evaluate Windows Server 2008, the 60 day evaluation period may be reset (or re-armed) three times, extending the original 60 day evaluation period by up to 180 days for a total possible evaluation time of 240 days. After this time, you will need to uninstall the software or upgrade to a fully-licensed version of Windows Server 2008.

If you wish to extend the initial evaluation and testing period, please go here and follow the instructions.

Register for a Guided Evaluation Experience
IT Professionals Register on TechNet Evaluation Center to receive Windows Server 2008 resources specifically selected for IT Professionals.
Developers
Register on MSDN Evaluation Center to receive Windows Server 2008 resources specifically selected for Developers.

Download Windows Server 2008 Standard
Download Windows Server 2008 Enterprise
Download Windows Server 2008 Datacenter
Download Windows Web Server 2008
Download Windows Server 2008 for Itanium-based Systems