A “virtual appliance” is a pre-installed and pre-conf application or IS running on a standard server or desktop computer in a virtual environment. There is a couple of hundreds
There is a couple of hundreds virtual appliances ready for download at vmware.com. For example anti-spam servers, mailservers, proxy, firewalls and IDS servers.
You will find Virtual Appliance here.
You can download Vmware server for free here.