There are hundreds of online directories specifically targeting lawyers and law firms. They range from the behemoths of Findlaw and Lawyers.com to the more practice specific ones of Divorcenet and LegalPointer.
There are two main reason to be listed in any online directory listing attorneys.
1. Visitors will come to the directory and find you
2. Your website will be listed in the directory giving your website an incoming link (having incoming links to your websites from reputable sources helps increase your exposure on the web).
The problem with reason #1, is that the directory listing is often priced to market demand not how many qualified prospects you will receive. For instance, Personal Injury often will cost 5-10 times more than a Divorce listing only because the PI attorneys are more likely to pay a high price. My advice, take a good hard look at the directory spot. Ask hard questions like "how many visitors and/or prospects should I expect to obtain from this listing?" Ask to see web traffic reports and ask some other lawyers that have that listing their opinions.
If you can get the lowest cost listing on a particular directory, then reason #2 would be a good move. Having an incoming link to your website for $10-$50/month from a legal directory is probably worth the positive effect on your firm's search rankings.
In conclusion, I would be very wary of spending big money on directory spots. If you want to spend big money, use it for more web content or look into pay per click.
Until next time,