Teaching, consulting, investing, building a product and charging for it. That's all that comes to mind right now!

Writing blog posts
Coaching
Consulting
Selling online courses
Teaching
...

I recommend listening to the ladybugs podcast, they have detailed information on each of those.

It's a lot of work but there is nothing so complex that you can't do it

ladybug.dev/episodes

The other suggestions + Software service plus ads, content + ads, 9-5, freelance, product selling/renting, etc

dev.to/tomavelev/ways-to-make-mone...

You can for sure, I do it (well used to).

However, keep in mind a few things:

1. Many people in tech will try to hype side business up because they are also trying to sell you something. I.e. ebooks, courses, 'mentorship', etc

2. Don't burn out. I used to do side-contracts and freelance all the time and loved it. However, at some point, I was just getting burnt out and I've dropped my side contracts all together.

3. Try doing a side gig that you enjoy doing. Often times, many of the 'dev influencers' you see selling things grew their following over time. Many of them started out doing what they do because they enjoyed it. Trying to write blogs or content on topics you hate is a miserable process, especially if you don't yet have a following and see no return.

4. Leverage your experience. Do you know what you do well? Others will probably want to know that information. However, just because people want it doesn't mean people will pay for it. Making online courses might sound good and many dev influencers preach this path but its a very competitive market (depending on the topic you choose).

First be recognizable or credible

First you can support on Open Source Community.
Like,

1. Involving in open source projects,
2. Answering questions in StackOverflow or reddit
3. Create Free Courses (on YouTube or any E-Learning Platforms) or Blog Posts

Then, You Can

1. Sell Online Courses
2. Teach One-to-One
3. Get Freelance Projects

Based on my Experience. Thank You!

Developing tools and platforms which become profitable.