As Keith writes - Microsoft® Hyper-V™ Server 2008 is a stand-alone product that provides a simplified, reliable, cost-effective and optimized virtualization solution enabling organizations to improve server utilization and reduce costs. It allows organizations to consolidate workloads onto a single physical server and is a good solution for organizations who want a basic and simplified virtualization solution for consolidating servers as well as for development and test environments. Low utilization infrastructure workloads, departmental applications and simple branch office workloads are also candidates to virtualise using Hyper-V Server 2008.
Hyper-V Server 2008 is a cost-effective solution that is convenient because it plugs into existing IT infrastructures enabling companies to reduce costs, improve utilization and provision new servers. It allows IT professionals to leverage existing patching, provisioning, management and support tools and processes. IT Professionals can continue to leverage their individual skills and the collective knowledge of Microsoft tools, minimizing the learning curve to manage Microsoft Hyper-V Server 2008. In addition, with Microsoft providing comprehensive support for Microsoft applications and heterogeneous guest operating systems support, customers can virtualise with confidence and peace of mind.
Go get it @ http://www.microsoft.com/downloads/details.aspx?FamilyId=6067CB24-06CC-483A-AF92-B919F699C3A0&displaylang=en
Now you may be a bit confused as to what Hyper-V Server is so please looks HERE to compare versions or HERE to see an overview of each version of Hyper-V server.