Good article from the Search Engine Guide entitled "Utilizing Popular Directories as Free Link Resources". It talks about how important it is to get listings from the search directories such as Yahoo, Open Directory (DMOZ), Zeal and a few others.
The obvious benefit is that you can get traffic from a directory listing from users who visit your site from the directory. In addition though it provides a valuable link to your site that can help build link popularity and also many sites will take links from Yahoo and Open Directory and place them on other sites, so you can get additional links pointing to your site.
Ive always noticed that sites that have BOTH a Yahoo directory listing and a DMOZ directory listing, do much better in the organic search engine results. Google has its own directory and its based on results from the Open Directory, so it really pays off to get that free open directory listing, but it takes a lot of patience.
Yahoo charges $299 per year for its listing and in most cases its worth it I feel. In the long run it will pay off and getting a link from Yahoo just seems to carry a lot of weight, in addition to producing traffic from the directory listing. For law firms, also getting listings from other known legal directories can be of great value, especially if those legal directories are well ranked.