I could try and state what those criteria might be. I've already mentioned a crucial one: whether you, being the "site master" like it or not. Another one is coherence, which means that it's better to stick with a few topics than to spread yourself too thin. A third one is whether the site seems to be of general importance.
Those three focus on the users - the surfer and designer.
One could also focus more on the actual medium. WWW has brought crossreferencing
to new, hitherto unforeseen, hights. But it also, by it's nature, involves
computers - and this means that it's especially suited to communicate material
unique to computers.
The following are good examples of this last mentioned aspect:
Dictionary of Special Plane Curves
The "Plane Curves" are such that they haven't been possible
to produce at such a scale and such a quality before.
The "Virtual Observatory" gives us access to views of parts of the sky of our choice, at different wavelenghts of our choice. Databases of this kind would be quite inconceivable without computers. (And their compilation would possibly be that too.)
Similar to these, and concerning something that I for some reason been interested in is this:
That page consist of links to sites with 4D applets and
such. Of varying quality I persume, but I haven't seen them all yet.
The "Virtual Reality Panoramas" might become common, or have is so already.
Rome AD 300
This particular one shows a recreation of the eternal
city. A contradiction of terms, but anyway.
(Takes some time'to load)
Educational applets can look nice. Exactly how pedagogical they are is maybe difficult to tell, but time could tell.
Interesting JAVA Applets
The above two are collections of statistics ones.
This Japanese site illustrates simple physics concepts. Written for a browser I don't use, so I haven't tried it.
Here are some simple but pedagogical applets illustrating
(It's from the University of Minnesota; the birthplace of "Gopher" - an important precursor to the WWW that wouldn't allow for anything more advanced than text.)
Animated Mathematical Physics
These might not be proper applets - they're simple animations.
The site is Russian, which means that the server is slow, and that they're
focused on theory like Russians often are. (My first tutor here in the
UK was a Russian theoretical physicist!)
So there is much stuff like this on the web. Of course you can use these more if you have them on your harddisk - but often enough it wouldn't be that useful, like when they show a simple concept you didn't know of but now immediatly grasp. So here one can face even more new choices, as if one hadn't enough already.
The animations in the following site aren't meant to be pedagogical, but to sell.
It shows how borders can be blurred, and categories stop being selfevident. Or something like that.