Several websites offer money (after a minimum level of earning) by clicking on advertisements. Hence, they are called paid-to-click (PTC) sites. One has to get registered before the project begins. Not all of these sites could be genuine, so be careful. One may also refer friends and earn money in the process. Some such sites are ClixSense.com , BuxP and NeoBux are some of such PTC sites.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.
Research individual companies in your desired niche: If possible, it’s always better to become an affiliate directly with a company (if they have an internal affiliate program), as no one else will be dipping into your commission rate. This is the preferred route for most of the prominent affiliate marketers, including Pat Flynn. Unfortunately, it’s also the most work, as you’ll have to do the research yourself to see who offers programs (they’re usually listed in the website footer).