As Paul Thurrot reports here, Microsoft have announced a standalone version of it’s virtualization product, Hyper-V.
Like VMware’s ESXi product Hyper-V Server 2008 (not to be confused with Server 2008 with Hyper-V which is the full version of Server 2008 with Hyper-V included) is going to be available to download for free within the next 30 days.
Have a look at Paul’s article for a few more details, including the release of System Center Virtual Machine Manager 2008.