Is there a product or service that you are particularly enthusiastic about? If you are, you may be able to develop a website that is built around selling it. You don’t have to be the actual provider of the product or service either. There are many businesses that offer these products and will allow you to sell them on an affiliate basis. For example, you may be able to sell a product on your website for a commission of 20% or 30% of the sale price.
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.