If you're in the market for x86 workstation virtualization products, commercially at least, there are 3 main players. Microsoft, previously Connectix, Virtual PC2007, VMware Workstation 6 and Parallels Workstation 2.2.
Virtual PC 2007 has one obvious advantage over the other two products, price! Being free certainly makes for a tempting offering, and indeed does make it a pain-free way to dip you toe into the virtualization pond. However, there is a price to be paid in terms of development and functionality. Since Microsoft's aquision of the product from Connectix, I don't think anyone would argue that development progress has slowed. Indeed my opinion is that Microsoft's purchase was a technology aquision rather than a product one. If your primary use for virtualization is testing Microsoft products, VPC is ideal. For more on Virtual PC, why not visit the Virtual PC Guy blog.
VMware Workstation has been around long enough to have achieved a good deal of maturity. Historically it was differentiated from VPC as VMware's angle was to produce a product which optimally virtualized a chosen set of operating systems, whereas Connectix chose to produce a purer x86 emulation. Over time I have personally owned (and here I mean spent my own cash!) both VMware Workstation and Virtual PC. VMware Workstation improves upon VPC's differencing disks with their implimentation of snapshots which are a very powerful feature and especially useful for running "what-if" scenarios on software configurations. VMware's ability create a virtual machine from an existing physical machine is a feature I've found particuarly useful. So if you need richer, more feature-laden environment for your testing then VMware is the way to go.
Parallels Workstation is very much in 3rd place at this stage. It's considerably cheaper than VMware Workstation and appropriately less feature rich. If you're tempted by this mid range solution, you can try out the demo before commiting your hard earned cash.
