If you aren't paying for a product, then you're the product being sold. Website "How Do They Make Money?" examines a number of tech companies, telling you how they make money -- and which ones might be tracking you to do it.
Some of these you may know already: Dropbox makes money by having a premium service it offers to users, while Facebook and Twitter make money solely through advertising. It's an interesting look at how your favourite tech services make money and who is relying on advertising the most -- and thus who might be trying to track you on the web. You can filter the list by revenue type or service type too. We only wish it was a bigger list, so we could see more companies that are trying to track and sell your data. Hit the link to see the list.