Hyper-V Server Release Candidate

Hyper-V Server 2008 R2 adds new features including live migration and expanded processor and memory support for host systems.

Microsoft Hyper-V Server 2008 R2 is the free standalone virtual server rather than the Hyper-V feature of Windows Server 2008 R2 - there's a good description of the differences between the two on the Virtualization info website. The fact that it's free and can be managed using the RSAT tools for Windows Server 2008 R2 makes this sound an attractive option, and raises the question of why you'd want to pay for Windows Server 2008 for Hyper-V if this alternative is free.

The main reason is that Hyper-V Server does not include licenses for guest operating systems as Windows Server 2008 Standard, Enterprise and Datacenter Editions do; Microsoft has designed it so you can run non-Windows workloads in a heterogeneous datacentre standardised on Microsoft virtualisation technologies. There's a useful table laying out the differences between Hyper-V Server R1, Hyper-V Server R2 and Hyper-V in Windows Server on the Virtual Boy blog.

The main improvements to the new version are:

Failover Clustering

The initial release of Microsoft Hyper-V Server 2008 did not include support for failover clustering. The addition in this release means you can manage 'unplanned downtime' - crashes - by failing over the problem server to another virtual machine in the node.

Live Migration

The addition of live migration means you can move running virtual machines between servers without any perceived downtime or dropped network connections. Live Migration essentially gives you the same option for moving servers that you get in VMware's vCenter Server. The previous version did not support any form of VM migration, which was a distinct drawback. 

Process and Memory Support

You get native support for up to 32 cores and up to 1TB of RAM on a physical system.
 
Updated Configuration Utility

Hyper-V Server 2008 is still command line only, so the configuration utility is designed to help you during system setup by avoiding the need to type long command-line strings.

You can download the release candidate from the Microsoft website.

Read Thomas Lee on Hyper-V

Check out why Server Core offers tangible benefits