"Donate" plasma.

Plasma is a component of blood, and the process is similar to donating blood. You're not permitted to actually sell your plasma, but you can be compensated for your time spent donating (and essentially, it's like selling your plasma). Your blood is drawn, the plasma is separated, and the red blood cells are returned to your body. You can make US$20-30 per visit and give plasma twice a week, but you must weigh at least 110 pounds, be between the ages of 18 and 59, and be in good health.^[1] Many college students do this to make extra cash.^[2] If you haven't been to the doctor in a while, this may be a good way to get a free health checkup.^[3