Hyper-V Server 2008 is Microsoft's free server hardware virtualization product. You can  deploy this Microsoft product  like a Windows product with your favorite deployment suite.

When you use Windows Deployment Services (WDS) or Microsoft Deployment Toolkit (MDT)to deploy Microsoft's Hyper-V Server 2008 however, you are prompted to provide a product key.

Don't be alarmed. You don't need a different license to deploy Hyper-V Server 2008 this way, but you will need a valid product key to install the product without being prompted during light touch/zero touch deployments.

This product key can be found on the DVD in the SOURCES folder. The file is called pid.txt
On the English Hyper-V Server 2008 DVD this file contains the following product key:

Q8R8C-T2W6H-7MGPB-4CQ9R-KR36H

Note:  
This product key will only work with Hyper-V Server 2008 media. It is not possible to install a Full installation or Server Core installation of Windows Server 2008 with this product key.

Tipjar: Reinder was the first to mention this little tidbit.