I've got around to checking the external links. The following have been removed or commented out where possible.
The page by Yoko Hasegawa about accent marks has gone.
(Commented out)
The lang8 website has closed down.
(Gone from FAQ)
The following website has gone:
My flickr account has been closed and the old gallery removed:
This was only a form of acknowledgement.
The "Kanji Clinic" is now defunct, replaced by some Indonesian-language thing:
Various other pages have moved slightly, and I won't list everything that has changed.
I think this is the first time in two years I have looked at this, hence the long list of problems.
I'll try to check these a bit more regularly from now on.
Please let me know about other problems with links.