I did this years ago (2008) for TalkBean (looks like they're still around). It was actually pretty great. At the time (again, 2008) I think they were just starting, and I got paid $20 an hour to be available, whether or not I had a student. I did 4 hour shifts every day, and probably taught for about half the time. I suspect the business model has changed.
It was very little work (in terms of prep / grading) because the classes were mostly conversation classes with Korean office workers. They'd get to their desk 30 minutes early and we'd chat. I'd prepare a couple of PPT slides of conversation topics / questions / vocab words, but these were very reusable w/ other students.
To be clear, though, I'm an ESL / EFL teacher by trade. Prior to getting this job, I'd taught for 6 years overseas and 2.5 in the US, and I had an MA. Incidentally, if you've got qualifications like that, another great online side gig is rating TOEFL exams. Again, it's about $20 per hour (or was in 2008) and requires no prep or grading. I'm not looking for extra income right now, but if I was (or when I want to go to PT work) that is the first thing I'll look into.