If you have a website or a blog, you should definitely sign up for Google Adsense. It’s one of the few programs you can truly ‘set and forget’ – once it’s there you don’t really need to do much else with it.
You can also insert affiliate links for products into these sites in order to gain even more income if you wish, but they are often known as Adsense sites simply because they are set up to attract visitors and click throughs on a specific subject.