There are thousands of “web directories” online, most of which are not worth submitting your site to. There are perhaps a few hundred directories that will be useful for search engine optimization, and only about twenty that will be useful for their intended purpose, which is to assist users in finding resources online; perhaps a few more, as I haven’t researched all of them yet.
Web directories are the easiest way for a new website to establish back-links for SEO purposes, and the better directories will send you visitors directly from their listings. These will be targeted visitors, in that someone who is browsing web directory topics related to that of your website will be ones who are likely to engage with your site, whether that means purchasing a product or otherwise making contact.
Some directories, such as the Yahoo! Directory and Best of the Web can be pricey, but they are valuable, nevertheless. Others are less expensive, and some are even free.
One way to choose a good web directory to submit your own site to is to browse the directory’s content, looking for a few different topics. If you are able to find what you are looking for, that directory is likely to be worthwhile. On the other hand, if websites are listed haphazardly, descriptions consist of keywords or marketing hype, or if there are a lot of empty categories, you might want to look elsewhere.