Affiliate programs can be an extra income source.

Can you make money with affiliate programs even if your join them for free?

Thought about an extra income? There are lots of business opportunities: Financial opportunities, inventions, mercahndise, reseller, second job, services, business start-ups, franchises, health and Nutrition, income opportunities, internet realated, mail order, MLM, residual income.
Affiliate Marketing is an Internet-based marketing practice in which a business rewards one or more affiliates for each visitor or customer brought about by the affiliate's marketing efforts. As always Wikipedia does a good job presenting definitions from different sources.
One definition of affiliate programs for affiliates or publishers may be:
Generate substantial revenue online through the promotion of affiliate programs. Browse through the free directory here and find relevant advertisers that you can promote.
Commissions varies and in this list they go up to 75%
Basically the steps for any affiliate program is:
Browse the list of affiliate programs
Choose one or more that you like
Register
Get the links
Promote
Receive checks!

Depending on your industry you can select those merchants that have products or services related to your field.

It sounds simple buy it's not. Like any other job it takes focus, effort, creativity, follow the rules, consistency and maybe more. However, if you are motivated enough and would like to study...
You can start at the Cotati Directory Affiliate Programs