Last week, Google introduced its new "interest based" ads, which is based on tracking your browsing activity and targeting ads based on that behavior. Fortunately, there are several ways to opt out.
There are obvious privacy issues that pop up when a company tracks your web history; it's also annoying and creepy. Fortunately, Google has implemented targeted ads in a relatively benign way, working with privacy groups like the Electronic Frontier Foundation to make avoiding these ads pretty easy. As EFF points out, the best way to do this would be to require users to opt in to targeted ads, rather than opt out.