Comment Approved Notifier sends an e-mail to your commenters when you approve their comments. It is a very simple plugin. There are no settings, options. It starts to work when you activate it.
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If you upgraded to WP 2.7 or WP 2.8, please, update v1.1 to v2.1
It requires at least 2.7. If you use WP 2.6.5, please use version 1.1
Thanks!