Based on the tests and interaction with the product, the only option that we would rule out is Parallels Workstation. Even though it is the cheapest of the non-free options, the trouble that it gave and the continual need to reset the VM immediately after powering it struck it off the list. Some of these issues would immediately be ruled out if you're using Parallels on the Mac OS X platform, where it is much more mature.
If you are a user of a Linux workstation and have a need for one or two Windows applications and do not need a full-blown virtualisation solution, then Wine is the best way to go. The applications are much quicker than they would be in a hypervisor, but do have the caveat that they may not work 100 per cent as you expect in Windows.
KVM finds its way in here as a hosted virtualisation suite, but is generally up against workstation-focused software rather than server software, which is its natural terrain. Due to its lack of performance and problems with graphics rendering it is not recommended for pure workstation purposes. However, if you don't mind tinkering with Linux and have a need to deploy multiple copies of the host operating system, for example in multiple Ubuntu instances, then KVM could work for you.
In the server room, KVM does find itself up against products more in line with what we covered previously, though considering that it is free, easily available on Linux and can replicate its host OS easily, it definitely has a place at the table, but is not the winner.
That only leaves two products in line for the title: VMware Workstation and VirtualBox.
The clear tear-away from the pack in terms of performance was VirtualBox. When VirtualBox works it is very good and only has issues on the rarest of occasions. The new 3D acceleration built into the product gives it a leg-up on the competition and the fact that it is free only sweetens the deal. A minor grey cloud on VirtualBox's horizon is the purchase of Sun Microsystems, the current developer of VirtualBox, by Oracle. It is unlikely that Oracle will kill the product, but could be tempted to merge it with its own virtualisation solutions or make it more enterprise-focused.
VMware Workstation is highly configurable and built by a company that knows virtualisation. During testing, it was faultless and the management systems were top notch. Although its performance stats were not the best, the feel of the system is not slow. Being able to record a VM session will definitely find a niche.
What is our final recommendation?
If you must have 100 per cent compatibility, certainty and would like support, then VMware would be your choice.
But provided you are not trying to boot NetBSD 5.0, we would recommend VirtualBox. It's fast, easy to use and has that sweet price point: free.