Even if you have a full-time job making decent money, I'm sure you'll agree that it's always nice to have additional income.  If you want to retire early, you need enough income to build up a sizable nest egg.
The good news is that there are a number of ways to bring in additional revenue.  Here are some of the best sources to bring in extra income...

Freelance Work

Upwork:  Upwork is a result of a merger between oDesk and eLance, resulting in the top freelancing site in the world. They offer jobs for designers, writers, translators, marketers, and much more.
Fiverr:  While Fiverr is a smaller network than Upwork, it provides additional freelancing opportunities in design, writing, marketing and more.
99designs: Whether you're a designer yourself or you want to hire a designer, 99designs is the best platform for design work.

Gig Economy Jobs 

DrivingUber is currently the world's largest on-demand rideshare service, but Lyft is right behind them.  Many people work for both companies.
Deliveries:  You can be part of Amazon's ginormous ecommerce system with the Amazon Flex program.  Or deliver food and other local goods with Postmates.
Pets:  If you enjoy walking dogs, then Wag! is for you.  Alternatively, Rover is great if you prefer pet sitting & overnight boarding.
OtherTaskRabbit lets you do all kinds of random tasks for people.  UrbanSitter is for babysitting.  If you're a chef, check out Feastly.  And HelloTech is for folks that can help with tech issues.

Sharing Economy 

Apartments:  If you own a house or apartment, you can rent the entire home or even just a room on Airbnb.
Cars:  You can also rent your car or vehicle on Getaround (get a $20 credit) or on Turo.  
Bikes:  With Spinlister, you can share your bike, snowboard, or surfboard.
Boats:  If you're lucky enough to own your own boat, you can even rent that out on Sailo.

Selling Stuff 

Letgo:  This app is great for selling electronics, games, clothes, and even big items like cars and furniture.
Decluttr:  Perfect for selling smaller items like cell phones, games, DVDs, CDs, and books.
BookScouter:  If you're a student, you can use this site to buy and sell your textbooks for the best price.

Notes / Disclaimers:

  1. The terms listed above are subject to change.  Always double-check the information on the company's website.
  2. Some of the links above are affiliate links.  If you sign up through one of the affiliate links we may get a small referral fee.  Please know, however, that we never base our recommendations on whether there is an affiliate offer.