Citrix High Availabilty
Some people will try to tell you that the only enterprise class virtualization out there is VM Ware, but I am here to tell you that Citrix is a contender. Not only is it a contender but it is growing in features and in acceptance. Citrix XenServer is actually the World's largest Internet Facing Virtualization environment, Amazon Elastic Compute Cloud (EC2). Because the foundation of XenServer is built on the open source Xen, there is lots of opportunity for developers to to build upon the feautres and functionality of the product. One such features is Everun by Marathon Technologies. The product will allow you to even unplug the server that your primary node is running on and the transfer to the secondary server is so seemless that if you were running XenApp on the server your end users would never even know that they swicthed to a different server.
http://www.marathontechnologies.com/high_availability_xenserver.html)