I think for a developer there are many channels to make money. What you call as a channel for "extra money" may be the major source of income for others. Here are a few things a developer can do:
- Freelancing (ofcourse)
- Consulting
- Advising companies and individuals on how to use certain technologies
- Making online videos
- Teaching people online through YouTube, Vimeo, etc. This can be a good source of income as well.
- Writing online articles
- Writing articles for sites like SitePoint, Smashing Magazine, etc. In fact, Hashnode is also paying authors for writing Hashnode Original articles. Read this.
- Writing books
- Publish books for Smashing Magazine or self publish at Leanpub
- Get a full time job 🤓