Wikipedia's co-founder Jimmy Wales gives an interesting business model keynote in which he discusses Wikipedia's business model and how advertising fits into that.
Wikipedia is a large online encyclopedia that people around the world can access for free. Wales says that Wikipedia would never charge people to use this encyclopedia, but they have to generate revenue somehow. Wales discusses how Wikipedia, much like Google, allows for relevant paid advertisements to be shown on the site. He talks about how advertisements and search engines seem to fit together well, so long as the advertisements stay relevant.
Jimmy Wales talks about how Google does a fantastic job with unobtrusive advertising. He says Wikipedia already partners with Google to provide ads on Wikipedia for this exact reason.