From Mozilla Labs:
Prism is a XULRunner based browser that hosts Web applications without the normal Web browser user interface. WebRunner is based on a concept called Site Specific Browsers (SSB). An SSB is an application with an embedded browser designed to work exclusively with a single web application. This application will be located in the Start menu on Windows. Fill out the URL of the website you want to turn into an application, give it a name and select where you want the shortcut to be created, then click OK.