I've been running a technical podcast focused on interview prep/insights for the last two years that has OKAY traction but is still a long way from making me any money. It's hard! It cost me money. About $500 a month right now, not counting my time. I enjoy doing it but I've certainly had tough months when I wanted to stop.

Doing freelance work has been the easiest way to bring in some extra cash as an engineer but damn do taxes really hurt that extra income.