Hello,
I have being a regular user of
http://www.saueregger.at/sudoku/HSH/HSH.htm.
It helps a lot (Nataraj diagrams) in avoiding mistakes when performing colouring.
Unfortunately, the whole site is down (second day without it).
If this is the end of it, I will certainly miss a lot.
Edit (Jul 9): sadly, it seems that it is really gone.
I hope that at least that clever way to display strong links isn't lost.