About a month ago, we announced that we were going to make a free version of Hyper-V Server (stand alone) available for free. As of today, the download went live and is now available. You can grab it here: http://www.microsoft.com/servers/hyper-v-server/how-to-get.mspx.
Please do keep in mind that this free version is not exactly the same as the one that comes with the full copy of Windows Server 2008 Standard Edition (and higher). The nice thing is that you don't need a Windows CAL to access the stand alone version unless you are running Windows Server within a VM on it. For those of you who really want to run Linux VMs or such on this free version are more than welcome to without having to purchase Windows CALs. Another thing to keep in mind is that for each Windows Server VM you install, you will need to purchase a valid Windows Server License.
Just to help out, the following is a snapshot of the table from the download site that compares features of this free version to the other ones.