Plus an important update about XenConvert
How to Convert a Physical PC or Server to a Virtual Machine | Physical to Virtual (P2V) in VMware
Update Oct 2017: Citrix no longer offers the XenConvert tool for download, nor does it support it. Your only options to convert a physical machine to XenServer are to first get it in VHD format (with a tool like Disk2VHD) and then import it. If you want to convert a VMware virtual machine to XenServer, you can try using the XenServer Conversion Manager feature.
Citrix Xen Server is a free enterprise-class server virtualization solution that is a direct competitor to VMware ESXi. Both are free and it is actually quite difficult to say which one is better.
Some people swear by VMware because of its longer experience with virtualization technology, while others say that Citrix has a better product. I don't know for sure, but in this post I'm going to talk about a feature that the VMware product does very well: turning a physical server into a virtual server.