Virtualbox runs in Linux, Windows, Mac OS. It is pretty fast and it is free. This is probably everything that you need for a desktop.

VMWare is good for servers. It is scalable. You can use VMWare converter to convert physical OS to a virtual image.

Parallels offers really nice solutions that are often overlooked. Although Parallels Desktop for Mac OS X is well known by a Mac community. And I see Parallels Virtuozzo Containers used more and more often on servers.

Unlike VMware's ESX Server or Microsoft's Hyper-V, which virtualize the hardware level, Parallels Virtuozzo Containers virtualize at the OS level. This approach has its pros and cons. On the plus side, OS virtualization offers lower overhead: It provides excellent performance and uses fewer host resources. All containers (term used by Parallels for virtual machines) share the same OS level. There is no need for an extra level of hardware virtualization. On the downside, all containers use the same OS. A container can be imported from a physical OS or created from scratch.