I started my secondary income by building a small tool to copy only changed files (search for "copy changed files" on google) and made it free and put it on a website and added a donate button.

I soon got my first donation, and one more, and one more and realized that I had made something that gave enough value to others so that they wanted to give something back.

This made me look for the next product to build, and it ended up being TSR Watermark Image - a windows software that lets you watermark images (doh)

This gave me a (nice) side income of around 1000$/Month until I attended MicroConf in Prauge where I learned enough to 5x that business.

Currently I am pouring the money from the watermarker into the latest product http://timeblock.com that will be the next step in my business adventure, hopefully becoming a SaaS app that can make a 5-6 figure MRR