Google places "relevant" ads next to just about anything using it's AdSense, a "content-targeted advertising program". You can even choose to have ads on your blog, with Google supposedly choosing the most relevant ads for your readers.
I don't think I'd make any money out of it, but I'm curious enough to be tempted to put the ads on here just to see what sort of things the program would come up with. It can be quite funny to look at the ads it chooses. Yesterday for some vague reason, I sent an email from my googlemail address. It included some Chinese characters, and Google started giving me all sorts links to sites in Chinese.
Today I received a reply which included a mention of someone who has just moved to Sydney. What ads does Google decide on? Firstly, an ad for "100% Australian Ugg boots", with express delivery worldwide. Then, a slightly more relevant cheap phone calls to Sydney ad. Lastly, an ad for aNewLifeInSydney.com, with the words "Sydney needs 50,000 people now. Start your new life today!", which struck me as quite strange. I'm not so worried about Sydney's population that I'd discourage immigrants, but how does Sydney need 50,000 people?