If you're in the northeast, ETS hires graduate students to grade general GRE essays.
In addition, my roommate is an SAT and GRE instructor + tutor at the Princeton Review. It's a decent hourly rate and not a lot of work. Consider tutoring with a company or on your own. You can advertise around your school or register on a website like wyzant.com. The more clients you get, the more you can charge per hour.
PS - I'd advise against mturk. On average people make about $5 an hour filling out surveys full time. There are way better options that you have as a grad student.