The obvious candidates are Virtualbox and Virtual PC.
But I have not been paying attention to VM's the last year, have I missed a good alternative?
VMWare player is also free to use, and can create virtual machines as well.
What's wrong with VirtualBox? That's still the best free one I've ever used.
I agree. I find VirtualBox has the best feature set of any of the VM solutions out there and its free.
I have yet to see another VM app support the seamless mode feature of VirtualBox which IMO is the one of the coolest VM features out there.
I install custom laptops for our users, many who need access to dual OS'. Since we started using a minimum of 4GB on laptops I've recommended virtualbox whether the host is windows or linux, the guest OS always being the opposite.
I use this myself, and all users I've set this up for have been very happy with the results, which are much better than dual-booting, although with lower performance, which is very seldom necessary anyways.