One of the things that immediately struck me about Stack Overflow when I came across it was how great the interface is. Everything is intuitive, advanced functionality "clicks into place" as you acquire it without major changes to the UI. The one or two very minor things which I find counterintuitive would seem perfectly normal on a lesser site.
As a (nominally) web developer I find this worth studying from a professional standpoint. We often learn best by examples (this is why "Beautiful Code" was so popular). So what are other websites out there that have really beautiful "just works" UI, and what makes their UI great?