While not a job site per se, GeoVisions have information about volunteering, working, and teaching abroad. The only downside about their organisation is that they require a fee. While I don't recommend paying in order to get jobs teaching abroad, the other programmes, such as volunteering, au pairing, or doing an internships abroad would be worth it.
I don't believe that you should pay in order to get a teaching job since there are so many job sites out there, especially for the countries that they target (China, Korea, and Thailand). However, with volunteering it's common to pay a fee.
For au pairing, most agencies also require a fee to make sure you have all the necessary documents, are placed with a good family, and are going to be a good au pair. As for internships, they only offer them in Chile and China, and unless your university has connections, I think it'd be pretty hard to arrange an internship by yourself. All in all, GeoVisions is a reputable company especially when looking for volunteer positions.
Got an idea for a job site?
Email me with your job site, name, and website (if you have one) and I'll post it ASAP.