While I don't personally have a need for a file comparison program I do find myself completely engaged by the site design for Kaleidoscope. Granted, I can see the usefulness of the app (particularly when comparing source code) but that's not the point I'm trying to make here - click around on the site - it makes use of great full page layouts with a nice range of style on each one but still cohesive throughout. I think I'm also a sucker for the sliding in-page style navigation and they embrace it wholeheartedly.