While regular submissions have fallen by the way side, directory submissions have become even more important.
There are two types of directories. The general kind (a portal that has many different types of categories and listings of web sites within, Ex. Open Directory) and specialty niche directories (a portal that has a database full of web sites pertaining to a specific industry, Ex. WebMD).
Links from directories are a big part in achieving good link popularity (the amount of relevant incoming links to a web site). It is also these links that will speed up the process of Google spidering a web site. Directories are unique in the fact that a link to a web site is always displayed, in contrast to a regular search engine that will only display a listing when related keyword queries are made to their databases. When a search robot like Google next visits a category within a directory, Google will easily follow the link to the listed web site. A link to a boat dealership inside the Business > Sales > Boats category tells any search engine spider that that web site truly is about boat sales. It is an immediate reference from a trusted source.
That is what link popularity boils down to. Search engines can no longer rely solely on the content in a web page to determine its overall relevance. If a web site is truly about boat sales, their reasoning is that other boating sites (or sites that list boating web sites) will link to it. It is relevance by association.
General Directories:
The most popular directory, Open Directory or DMOZ, feeds its results to many popular search engines including Google. Other general directories also get many people searching by category.
Specialty Niche Directories:
These types of directories provide more value in terms of link popularity than they do in actual traffic. However, studies have shown that a user who finds a web site through a lesser known, but more narrowly focused, directory is more likely to buy a the web site's products or inquire about their services.
