The folks at Search Engine People have some interesting thoughts whether to go for the big 3 (Yahoo/business.com/botw ) or some other well maintained directories. Here is the article they wrote some time back - ( check all the directories as some might have gone out of SERPs) http://www.searchenginepeople.com/blog/top-25-most-powerful-web-directories-list.html
Directories are good to boost up the traffic if it is of good quality.. there are many high quality paid directories. free directories are spam farms & have no benefit for SEO.