The Advisers Group

A virtual appliance is virtual machine image designed to run on virtualization technology from vendors like VMware,  Xen, or Microsoft.
Virtual appliances eliminate the installation and minimize the configuration of software applications because the vendor does this for you. Many virtual appliances have a web interface to configure the appliance.
A virtual appliance is usually built to host a single application, and so represents a new way of deploying network applications.

VMware and Microsoft have an extensive listing of Virtual Appliances on their websites.