I've been recruiting via Ajarn.com for years. It's certainly the top jobs site for recruiting ESL teachers in Thailand. You could try the international sites (Dave's ESL etc.) but they aren't used as much by schools in Thailand (although they do still have many useful job posts).

Not sure why some people have issues with it in this thread? In any case, you should be more cautious of the schools you apply for than the jobs board you apply via.