R – What web sites are available that contain good examples of user interface design

Someone had to say it: Stack Overflow!

Seriously - it may not be immediately beautiful to look at, but for what it is, and for its target audience and how they use it, it's a great example of both visual design and interaction design.

The layout, the use of colour, the sensible use of AJAX, the lack of fancy graphics, all give a good experience IMHO.

It's also a good example of how to incorporate advertising in an effective but unobtrusive way.