Personally I use it for gmail, google docs, google calendar, outlook web access, and stuff like that.
Following are some helpful introductory links on prism:
- http://prism.mozilla.com/ - the prism home page (it's a mozilla labs project)
- https://developer.mozilla.org/en/Prism/BundleLibrary - the library of webapps that people have already created as prism apps. Your mileage may vary - using prism as a stand alone app has brought me the most success on linux with the current version (1.0 b2).