Any side income earners here?

Have you worked/working on something that earns you additional income over your salary?

What do you do? Why do you do it? How was the journey? Do you enjoy it? Is your work aware of it? How much do earn from it?

I will start with myself. I worked on reverse engineered camera for Android devices, for 2 years, I did it because there was no camera for my phone with custom rom that used all the features of the hardware at the time I started the project, It started with an insult on a noob question XDA. At the end of two years it had downloads over 10K. It got famous in related communities. I even got a phone from the OEM for further development. Made a lot of friends Indian as well as international. It was fun for an year. After 2 years it become a game of cat and mouse. The manufacturer took steps to make my efforts to unlock features on older devices difficult. I lost motivation. Felt a lot like unpaid labour. Got tired about bug reports. My bosses were aware of it. Earned around 1.5L from donations and ad revenue.

Now I am trying indie game development.