Ask HN: How to come up with monetizable side project ideas?

I love building small tools which solve problems I myself face in my day to day life. Having been worked in different companies over the last 4 years, I feel I've acquired enough knowledge to develop big applications(big enough to make money) on my own.

I was wondering, how one can come up with side project ideas which can generate few hundred dollars per month on the side. Any inputs, resources and wisdom are appreciated!