rPath (www.rpath.com) is a company that provides a toolkit for building virtual appliances. These are virtual machines with an operating system and application set preconfigured, and designed so that user can download them and run the application with minimal configuration. This is particularly useful for checking out different server applications, without needing to know how to get them all setup. You can checkout the Virtual Server / Virtual PC virtual machines here.
I had fun looking at this Wiki virtual appliance – it boots up with no configuration and tells you the URL to go to to configure and use the wiki.