Many sites receives most of the traffic from search engines (especially from Google). Ryan Stewart's blog has been delisted from Google and he describes the consequences.
Yesterday was one of the scariest days I've had in a long time and it put in total perspective the power that Google wields. (...) I was working on a blog post and was searching for something I'd written before so that I could reference it. Instead of using my blog's search, I of course used Google's site feature, like I always do. Only this time, I got nothing. (...) I was confused, angry, and I felt completely helpless. Everyone uses Google. It was a big referrer to my blog and when I posted tips and tricks for Flash/Flex/AIR I did so knowing that someone could Google the problem and find my blog.
In his case, it wasn't Google's fault. "It turns out that Ryan's blog had been hacked, and Google does remove hacked sites from our index to protect our users," explains Matt Cutts. But whether a site is removed from Google's index accidentally or on purpose, it's interesting to quantify the importance of a popular search engine like Google for both users and publishers.