Today VMware released version 4.0 of Fusion, a popular option for running Windows (and other operating systems) on a Mac. VMware Fusion 4.0 offers hundreds of new features. The big news is full compatibility with Mac OS X 10.7 Lion, which for example makes it possible to add Windows applications in Launchpad, see them in Mission Control and switch between them using the multi-touch gestures.
You can for example have three virtual machines running Windows XP , Windows 7 and OS X Lion simultaneously. Better performance by up to 2.5 times faster 3D graphics over the previous version is also in the bargain.
It is also now possible to also run Mac OS X 10.7 Lion or Mac OS X Server (Leopard, Snow Leopard or Lion) in a virtual machine. Or you could migrate your entire Windows installation over to your Mac, as seen here: