Prism is Mozilla Lab's solution for running web based applications outside of a normal web browser interface – and is an example of Single Site Browser (SSB).
Why would you want to do this? Through AJAX and other emerging web technologies, web applications are increasingly behaving like desktop applications, e.g. Google Documents, Flickr etc. Normal web browser controls (backwards, forwards, refresh buttons etc) no longer have a direct correlation to the web application which is generally being controlled from the browser content itself. Inside Prism links to external sites are opened in new windows, so that the application never gets lost.
This is how Google Mail for mobiles, Google Maps for mobile and similar downloadable mobile phone applications work. Such web applications appear to run as standalone programs on the mobile phone, which can be linked to directly from phone menu and which benefit from a standard interface with out custom tool bars and settings.
Available for Windows now, with Apple & Linux versions on the way.
http://labs.mozilla.com/2007/10/prism