What's the best webapp UI you've seen and what was used to make it?
Travis: Seems like most comments here talk about the "intuitive" aspects of the design.  Which I agree with -- an app should ideally be intuitive.That said, being intuitive really only is an issue during the user's learning curve.  As every vi/emacs user knows, sometimes you can be way more productive once you adapt to the computer, rather than the other way around.  Gmail, for instance, has a very poor UI from a traditional perspective, but I find it very easy to be productive once I learned the keyboard shortcuts.Intuitive is generally a good thing, but in and of itself isn't the most critical requirement for being usable, IMO.  I prefer clustering of concepts, simplicity of presentation, and workflow optimization much more important.But I still try to design my apps to be intuitive.  After all, why not build on common concepts?