Microsoft has just released Virtual PC 2007 as a free download.
Virtual PC is a client for hosting virtual machines. I have written before at how valuable I find virtualization to be, for development, testing, evaluating, and of course managing my little home infrastructure. Community Tech Previews of the next version of Visual Studio, code named 'Orcas,' are distributed as virtual PC drive images, which I find really convenient.
Virtual PC 2007 fully supports Vista both as a guest and a host. Some other key features in 2007, lifted from the Virtual PC blog, include:
Visit the Virtual PC 2007 site to see the demo, read the application compatibility whitepaper, read the technical overview, or just download away!
If you're interested in virtualization, my colleague Dave Northey writes a lot about it from an infrastructure perspective.