Google bombs
Various friends have pointed me to the notion of Googlewhacking, but I haven't seen much discussion outside of the geek press (Slashdot etc) of the idea of "Google bombs." It turns out that Google indexes not just by words that appear on a page, but also by words used in links to that page. So if a whole bunch of people use the same relatively uncommon phrase as the text of a link to a given page, Google searches on that phrase will be more likely to bring up the page. Thus, a large enough mass of people linking to a page in a particular way can influence Google's results for that page.
There are all sorts of interesting ramifications; see the article at Microcontent News for more info.