On the side I sell budget star tracking astrophotography mounts I designed and build in my basement. I also do a few consulting gigs each year. These efforts combined bring in an extra ~25k annually. My plan is to develop two more space-related products, hire a tech, and transition to more of an oversight and design role rather than the one guy running the whole thing.
For the record I don't think most people should do this, it is not a balanced life to work this much, generally speaking. But for me it's a worthy discomfort to relieve financial pressures from student debt and being the only breadwinner of a young family. Plus it's fun stuff, I get to design cool products which is what I do for fun anyway, I just usually stop at the level of prototype when it's just for me. ;)