The directory industry has grown tremendously in the last couple of years, and there are many, many directories out there. Some are very good, while others are still in their youth. Internet marketers and SEO folks often use directory submissions as part of their plan to promote a site.
If you recall, Yahoo started out as a directory in the 90's and has since grown to become a tremendous portal. I have seen some directories follow a similar path but on a much smaller scale, and it has been very interesting to be a part of this industry.
This site is powered by PHP Link Directory, which is the most widely used directory script on the net, and powers something like 50,000 sites. If you haven't already seem it, you should check it out.