Website Review: buddyschool.com
In short: Students can find teachers and teachers can find students. You can actually learn or teach anything, but English is the most popular. Most lessons happen using Skype or another video chat service. Prices for the lessons seem to be from $5-$50 per hour. Students and teachers decide what the lessons will be about.
For students: All the teachers have ratings, so you can choose a popular teacher if you want.
For teachers: They encourage you to buy an ad on the site so that students will notice you. It might not be a bad way to go at the beginning. They have flexible pricing that lets you pay, for instance, $10 for 100 clicks on your profile.