Have you ever had a web site "go dark" on you? or found that a given page on a site somehow disappeared? Maybe it's only temporary (there may even be a "we're down" message, though the site or server may just fail to respond at all), or maybe the failure of the page or site will be permanent.
The good news is that there are at least two easy ways that you may well still be able to see that content you may be missing: the Google cache (to at least see the last version which Google may have cached), and the internet archive "wayback machine", which often lets you see YEARS back in the history of a page or entire site, including one that may be long-gone.
In this post I share tips (and gotchas) on using both tools.
They aren't GUARANTEED to have the page you're looking for, but I find that they do about 99% of the time I try them (and I use them a lot, because I'm often mining gold in old blog posts or articles which have gone away across many sites I have visited).
[Updated June 9 in a variety of ways, mostly minor, but with some additions in the "trip down memory lane" discussion.)