In the world of website promotion directory submission is very important due to following reasons.

[1] They will provide you strong back links
[2] improve your popularity
[3] generally they are permanent linking. so you will not see your URL dancing in SERPs

There are some common rules for directory submission. Follow these rules before you submit your site. other wise your site may be rejected by editors. The rules for submit your site in directories are..

[1] They never accept mirror sites.
[2] Sites with overlapping and repetitive content will not accept.
[3] They generally don't accept sites that redirects on other link.
[4] Sites with 'Under construction' status will not accepted
[5] Affiliate/referral links generally not accepted.
[6] Don't use promotional language.
[7] Don't write your title with all words in CAPITAL
[8] submit your site in appropriate categories

Some of Top Directories

[1] http://www.dmoz.org

We can say dmoz is father of all other directories. your site will include in about 1200 other web directories including Google. you will see in boost in your PR once your site accepted in dmoz.These ODP links are treated like gold by Pagerank™. It doesn't even matter what the individual PR of the category page is. I have seen sites gain a large PR boost on the toolbar as soon as Google updates it's directory with the latest one from dmoz.org

[2] http://zeal.com/

If you site is non commercial type than you can submit your site to Zeal.com. which is actually a Looksmart directory. just like Google obtains results from ODP, Looksmart obtains it's non-commercial listings from the Zeal web directory other wise you have to submit your site in looksmart directory which is paid option

[3] http://dir.yahoo.com/

like zeal if your site is non commercial type than only you can submit your site here other wise there is paid option available.

[4] http://joeant.com

Become an editor to add your site. paid inclusion option also available click here to know morehttp://www.joeant.com/suggest.html

[5] http://www.gimpsy.com/gimpsy/searcher/suggest.php

Only paid option is available to submit your site.nice directory and provide strong back links

[6] http://www.goguides.org/info/addurl.htm

Only paid option is available to submit your site.nice site and provide strong back links

[7]http://www.skaffe.com/info/addurl.htm

Become an editor to submit your site in Skaffe or paid inclusion option is available.

[8] http://www.wowdirectory.com/

You can submit your site here both free as well as paid inclusion (for fast and guaranteed review) are available.

I have created a Large list of web directories you can visit it here http://seo-resources.inetzeal.com/