For the non-geeks present, a 404 page is the page displayed when you click a hyperlink but the linked resource (the page, image or whatever) is unavailable. 404 refers to the status code in the HTTP spec. Here is a much finer definition, Strong Bad style. CNN did a piece on 404 pages a couple of years ago as well.
I was mildly amused, then, to see Greenpeace’s 404 page, which features a dodo bird in the top left corner, and a number of possible explanations for the missing page. The first one reads ‘the page may be extinct, like many whales, chimpanzees, and gorillas in the wild could be without your help.’ Never miss an opportunity to lobby your target market, I guess.