Tutoring organic chemistry for cash, yields an extra $2500 per year.
Haha everyone I know hated that fricking class.
Things I do for extra cash: Look at craigslist "GIGS" section and "etc" tab under jobs. There is always someone that needs a couch or some shite moved and I have a tahoe that can move just about anything. Also other random shite is always on there "I need a ride across the bridge" or" someone to take me to BR" and I can pick up $50 for a couple hours of work doing anything from moving someone in to serving food at an event.
I also bet sports and happen to win more than I lose so that helps, not for everyone though.
I'm still in grad school so I have more opportunities to make side cash than someone with a career already.