A list of directory submissions with PR value (manual submission for each of your properties):
For complete list: http://www.onewaytextlink.com/links.php?type=free
Submitting your domains to directories is ONE valid way to get backlinks to your sites. There are paid services and free services. Concentrating on the relevant directories to your field will help insure quality backlinks which will be noted by the major search engines.
Directory submission, pinging, social bookmarking: learning how to use these tools to help other people solve the problems they have will catapult your services further on the web. Regardless of your niche or business, the key to success lies in helping as many people along the way you can.