A good way to earn residual income is to have a website such as this one utilizing affiliate links (this one doesn't yet) to generate passive cash. Affiliate marketing consists of connecting people to products and/or services, and in return, receiving compensation for each conversion. Once you find a product or service you like, search to see if that product or service offers an affiliate program. In my opinion, affiliate marketing is the best way to make passive cash and the earning potential is limitless.
Amazon is a major player in this game and you can easily sign up to become an Amazon Associates partner.
Below is an example of a banner that links directly to Amazon.