If you have your own website or blog with good number of visitors, then you can earn a good amount of money by placing advertisements on your website. There are some third party websites what serves ad on your webpage and pays you. All you need to do is just register with one of them and after logging into your account you will get some HTML ad code. Then place those codes on your webpages and ads will start appearing. The thirdparty website will pay you on the basis of how much visitor views and clicks on those ads. Finally after each month you will get paid via check, paypal or other payment systems.