If I'm looking to buy something and don't know where to buy it, the first thing I do is a Google search followed by opening all the ads above and beside the results in tabs.

I've also been captured by retargeting campaigns by web services. Quite a few YC-funded companies run them. You only see their ads after you've been to their site... but they follow you all around the web, everywhere AdSense/DoubleClick reaches, which is 90% of all display advertising.