There are a number of these services available on the net, the one I recommend is New-List.com (free). These directories will show your ezine description or ad to anyone looking for information on a given subject.
Another good spot to look is DirectoryOfEzines.com (fee-based).
For one thing, submitting your ezine to ezine directories will create links back to your web site or blog. It will also generate traffic of highly qualified prospects who are very likely to be interested in what you have to offer. At the very least they will sign up to receive your e-newsletter, and from there it’s up to you to convert them to paying customers or clients.