They're not all free, but this is a pretty good list - http://www.strongestlinks.com/directories.php. Actually many SEO experts suggest submitting your site to all directories that provide static links, fee or no fee. Anyway, you can see the fee for each directory on the right. You can also search for directories in DMOZ to find specialized ones. Or do a Google search for your key word + directory. For example "ESL directory".
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