For years, Microsoft has had iSCSI target software that lets a Windows Server act as shared storage for clustering. However, this software was only available as part of the Windows Storage Server and you couldn't use it in production environments (unless using an actual Windows Storage Server). As of April 2011, the target software is now available for free and supported in production use!
You can use this for:
highly available VMs
clustering of file and print services
I know I'll definitely be using this in classes where we do high availability of VMs. Not sure about the production side yet.