The dead link in your particular example has been preserved by the Internet Archive. The Internet Archive is a fabulous project, and if you don't know about it you should spend some time getting acquainted.
However, looking at the link, it appears to be a student poster presentation from an AAPT meeting, whose "formal" citation is probably only an abstract in a meeting program from 2001. I made some conference posters around that time, which I may or may not have thought to put on my squiggle-username university student website, and which therefore may or may not still exist anywhere to be retrieved using a citation. (One of my posters from that era survived for a surprisingly long time as a letter-sized printout on my mother's refrigerator, but I think it was replaced by some mushroom people drawn by my own children.)
As a comment says: an answer which isn't useful without its link is not an answer, for exactly this reason.
Beware also that some links are better than others.
For publications, a link to a digital object identifier may be more useful to users of this site, especially casual users, than a traditional citation. A person who doesn't know what to do with "Phys.Rev.Lett.867-5309(Jenny)" will almost certainly know to click on the underlined text, and the digital object identifier is supposedly stable even if the publisher updates their internal linking scheme.
Links to the arXiv preprint server also likely to be stable long term (though if you notice a link to
arXiv.org/pdf, please edit it to
arXiv.org/abs, so save casual clickers the pdf auto-download).
Wikipedia is great, but beware that your Wikipedia link won't say quite the same thing in five years as it does today.
Youtube URLs seem to be mostly stable, but videos do sometimes disappear. Video links are also less accessible than text links, because they're mostly impossible to skim. We mostly don't have the problem of people trolling with links to irrelevant music videos, but there is always the risk.
Links of the form
school.edu/~squigglename, as in your example, are practically guaranteed to vanish when their maintainer graduates or moves to a new position. There are a few exceptions.