Virtual Appliances are nothing more than preinstalled virtual machines. Due to obvious licensing limitations the majority of public virtual appliances are Linux based. While the concept is very nice and very useful for software demonstrations, due to the considerable overhead of guest OS until now it didn’t catch with home users.
At this moment all 3 major players in desktop virtualization are either offering virtual appliances or maintain an appliance marketplace.
VMware Virtual Appliances
Microsoft Virtual Appliances
Parallels Virtual Appliances