When it comes to “off-page” factors (links) for SEO, their importance cannot be overstated. A mix of natural, relevant links is always a great way to go, and it’s a secret to good rankings.
You need to Always Be Link Building, with no spam. The more links, the better.
This is something you can build by offering not only good quality content (people will natually link to you), but research web locations where relevant links can be had. You should look first at your competition to discover where they get their links from and what quality sites they belong to–as it will help guide your link-building efforts. You may find that they belong to directories.
A fast, easy way to check for link popularity of a site is to use the link: command in Google (limited) and preferably linkdomain: in Yahoo. Yahoo will show you direct links for the domain, example: linkdomain:zillow.com (more than 2 million links).
This research is important, and there is another, complementary (read: additive) way to get links to your website. It’s a form of paid links, but Google tends to value them, especially the top sites.
Since a good way to track the lists you submit to is via Excel Worksheets–I have pulled data from strongestlinks.com and broken them out into regional/subject tabs as well as global data sets on the search marketing resources site.
The worksheet lists PageRank, Alexa Rank, Fees, and more information.
===> You can download the excel sheet here.
Since this is an exhaustive list, I don’t recommend signing up for all of these–but selecting the top directories in your niche (do the research first), adding key directories every other week or so, and not overdoing it. There are free directories, but many are paid–and if you have commercial websites, most require a fee. You should sign up to get more features enabled, including searching.
I recommend DMOZ, Yahoo, Business, JoeAnt and Gimpsy to start.
You should also investigate Back Link Watch for more links detective work; and SOLO Seo Link Tool provides a powerful way to manually use search builder to find and add links using Google. A Bad Neighboorhood Links checker may be a place to review for poor link networks.
Make sure to not forget the social media, videos, blogs and more. You are ultimately doing all of this for one reason: to build an authority asset(s) on the web, one that generates trusted traffic and converts to business, 24/7, 365 days a year.