In addition to a client trial version of Windows 8 Enterprise, Microsoft have also offered up a trial version of Windows Server 2012.
It’s called the Windows Server 2012 Release Candidate (RC) and it’s available both via Technet and MSDN.
Windows Server 2012 offers businesses and hosting providers a scalable, dynamic, and multitenant-aware cloud-optimized infrastructure. It securely connects across premises and allows IT Professionals to respond to business needs faster and more efficiently.
Windows Server 2012 offers developers a broad, scalable, and elastic server platform. Using a common set of languages and tools, it provides the flexibility to build and deploy applications and web sites on-premises, in the cloud, and in a hybrid environment.
In addition, you can find virtual labs for Windows Server 2012 at Technet.