If you have a blog or other type of site, you can build affiliate links to different services on the website. Many people use Amazon as an affiliate partner. For example, if you are a beauty blogger writing about different products, you can set up Amazon affiliate links on your blog so that whenever someone buys the product you mention on Amazon, you receive a percentage of the sale. Amazon is not the only affiliate partner out there. Here is good, in-depth information on affiliate marketing. 
Do you know of a successful business that needs capital for expansion? If so, you can become something of a small-time angel investor and provide that needed capital. But rather than offering a loan to a business owner, you instead take an equity position in the business. In this way, the business owner will handle the day-to-day operations, while you will act as a silent partner who also participates in the profits of the business.
After employment, I think that most individuals gain income diversification through investing.  It is important to look at why we invest: because at some point we plan on using this money for something.  For most, it is saving for retirement, and the investing is done through vehicles, such as a 401(k) or IRA.  But investing is not just about stashing money away for a rainy day – that is what an emergency fund is for.  Investing is about having enough capital to generate income.
There are a ton of ways to diversify your investments, some of which can send real income your way. By opening a brokerage account and investing in ETFs or mutual funds, you can earn real returns you can use to supplement your income. Of course, the flip side can also happen – as in, you can lose money. So, make sure you understand the risks before you dive in.