VMware, the market leader, can command the top price (look at Red Hat). The other leading product is XenSource, but it is not cheap. Both VMware and XenSource offer free solutions, but they may not be what you are looking for. Since XenSource is open source, you have the the option of implementing and solving your support problems by using your own tech abilities and ingenuity.
As you look over your options, keep in mind that Red Hat will be shipping Xen technology and management tools in RHEL 5. SUSE and Mandriva are also said to be working on including Xen technology, Sun is bullish on Xen for its products and Windows is also getting into virtualization. Some virtualization efforts are looking not at virtualizing an entire machine (the way VMware does), but at virtualizing applications.
The entire virtualization field is complex and in flux, so be sure to begin your research with a list of your requirements (chips, OSes, host/guest configuration, etc.) so that you can see what solutions come closest to what you want at the price you want to pay. And somwhere down the road, there may be a common hypervisor solution that lets more systems enter virtualization more easily.
This was first published in January 2007