I won't say the companies, because I don't necessarily want my full time job knowing about the other one.
Full time, I am an Application Engineer that works on the design and application for industrial furnaces in combustion and metal heat treat (helping companies lower their emissions, become more efficient, etc).
Part time, a "local" steel plant pays me very handsomely to troubleshoot and fix their large industrial furnace on weekends. They pay me handsomely (about 3x per hour what my regular job is, if it was hourly).
I make about as much in just working 2 Saturday's a month as I do in about 1.5 weeks at my regular job, which already pays well. So its almost like an extra whole paycheck (because health insurance, 401k, etc doesn't come out of it) for only working 2 days vs a 2 week check.
Have only been doing it for 3-4 months, but it has paid off my car and it will pay off my wife's car next as well as our whole family going to the Caribbean next year. It has really been a "God send".