An off-topic question asked at Mathoverflow by Andrew Stacey; but one which fits here:
I'm interested in hearing of examples of mathematical (or, at a pinch, scientific) websites with serious content where the design of the website actually makes it easy to read and absorb the material. To be absolutely clear, the mathematical content of the website should be on the website itself and not in an electronic article (so meta-sites that make it easy to find material, like MathSciNet or the arXiv, don't count).
Edit: I'm extending this to non-internet material. I want examples where the design of the document/website/whatever actually helped when reading the material.
As a little background, I know that LaTeX is meant to help us separate content from context and concentrate on each one in turn, but I often feel when reading an article that the author has concentrated solely on the content and left all of the context to TeX. This is most obvious with websites where there are some really well-designed websites to compare with, but holds as well with articles.