I've really enjoyed tutoring through Wyzant. They have a comprehensive system for getting approved for new subjects, lots of job listings for online and in-person lessons, and great customer service by phone. Their payment system (wait until payment information is on file to schedule a lesson, and everyone has to approve a rate change) allows me to conduct lessons without awkwardly having to take time out to discuss money. I also like the system by which students can review their tutors, and tutors are able to share selected reviews with new potential students.
Difficult things about tutoring through Wyzant: You can only take each certification test once, so make sure that you study up. They're short, which means they don't take much time, but if you miss a few questions (for instance, on adjective endings, in a foreign language test) you can never tutor in that subject. Most postings you apply to won't get you a response- I've been applying to about 15 jobs a week through Wyzant for a month or two, and have only gotten responses from 9 students (3 of whom have become students.) That means it's not great as a full-time job. I also rated it 3 stars for compensation/benefits, because you will never receive the full amount you are charging. You will receive 60-80% of your rate charged, depending how many hours you've tutored. That means that if you charge $30 an hour, you start off only making $18 an hour, despite the fact that your students are paying $30 per hour plus a service charge to Wyzant. Still, $18 per hour is better than a lot of other jobs, and Wyzant allows you to adjust the rate charged for an individual student or your default rate.
Their system allows me to connect to students online and in person, arrange a time to meet, and not worry about payment. It helps me show of my expertise and become approved (or show interest, if I don't have the time to get approved yet) in as many areas as I want. I never have to worry about a student not paying me for a lesson or be concerned about how to find opportunities. Overall it's a very good system, and has helped me tremendously.
***FOR POTENTIAL TUTORS***: Get approved for as many subjects as you can, sign up to tutor online and in person if possible, and study for every subject test before you take it. Happy tutoring!