Software consulting. You’d be surprised how many little businesses out there have a need for custom software. I make about $60-70k/year in my spare time (maybe 10 hours/week) doing everything from adding features to legacy products, building brand new applications and even simply conducting technical interviews. It’s also pretty nice to work on something besides the usual thing at work, experiment with new/different…
It was fun to work on something outside my day job but it required some judiciousness to ensure I could meet my set time limits.
I found taking on bigger projects + a demanding day job pushed me to burnout pretty quickly.