Since popularity has become increasingly important for obtaining and maintaining search engine visibility, being listed well in directories (also known as human-based search engines) and industry-related sites is essential. In fact, most people often begin search engine marketing campaigns with the directories. It's one of the quickest ways to give your site credibility.

There are many directories available on the net and each has its own set of conditions, pricing and service. Some directories charge submission fees. However, paying these submission fees does not guarantee that your site will be listed in a directory. Rather, it guarantees that a directory editor will review your web site within a specified period of time.

Yahoo's business directory and Business.com offer a straightforward paid submission program.

Submission to the Open Directory is still free.

LookSmart is a little different. They offer a pay-per-click program in their directory. Every time a user clicks on a link to your site, you pay 15 cents per click. But make no mistake - LookSmart is a directory. To submit, you still must select a category and write a clear, concise description.
It is very difficult to modify a directory listing after initial submission. So do it right the first time.
Directory submission is only the beginning of a link development campaign. To be successful, sites should have links to them from other reputable sources. A qualified search engine marketer can help you build quality content, create a link policy, and manage link requests. Moreover, expert directory submitters can do the job much more quickly and effectively.