Although by their nature VM operating systems don’t require their own software, a number of companies are making applications to work with them. Generally these are designed to make life easier for the administrator in various ways.
For example, Invirtus’ VM Optimizer reduces the size of Windows virtual machines to make the images easier to transport and share. ToutVirtual’s VirtualIQ suite is designed to make virtual machines easier to manage. Vizioncore offers products to do hot backups and disaster recovery on VMware ESX Server.
Some of these products are limited in which guest operating systems they’ll support, and they may also be limited in what hypervisor they’ll support. At this writing, the most commonly supported guests are various flavors of Windows and Linux, and the most commonly supported hypervisors are VMware and Virtual Server.