A comprehensive list of free business directories is on HubSpot's blog under the title "The Ultimate List: 50 Local Business Directories" by Eric Vreeland. The site offers other resources such as a link to a Firefox autofill add-on to make it easier to create multiple listings.
Vreeland's list includes directories for the most used Web search engines, online yellow page listings, directories by location and local directories. It is recommended you use as many directories as possible to optimize your opportunities for being found by online searchers. Vreeland also offers a free e-book to learn about optimizing search engine practices from experts on current marketing techniques. Use this guide to learn how to drive traffic to your business with web browsing. Direct links are provided for business directories like Google, Bing, Yahoo and Yelp.
Links for updated versions of the Yellow Pages online directories are provided for YellowPages.com, Whitepages, Yellowbook, USYellowPages and many more. There are links for business directories by location such as CitySearch Mapquest, Local.com, TeleAtlas and Navteq GPS. There are also links to many local directories such as LocalEze, EZ Local, Discover our Town and more. This resource includes direct links that take you to the business directory listing pages.