ODP (Open directory project) is the largest human compiled directory and if you can get listed in the DMOZ directory, your ranking will improve in google as well as other search engines/directories like AllTheWeb, Excite, Teoma, WhatUSeek etc.
DMOZ submission is free but it's not easier unless you are not carefully follow their guidelines.
Category selection, Title and Description are the most important inputs to this directory. some tips below:
Category selection
Go to directory home and select suitable category and dig to the appropriate sub category. Check the existing listings and make sure your site fits to that category. Submission to wrong category may delaying/cancellation of your request.
Title
1) Do not use All caps
2) Use the first letter of each word in capital
3) Never use your URL as the title
4) Never use special characters including punctuation
5) Limit your title below 70 characters
4) more tips: http://dmoz.org/erz/sites/title.html
Description
1) It should be descriptive and concise
2) Do not stuff your keywords
3) Do not use sentence like 'we are the best...'
It will take atleast 3 months for the inclusion of your site. If your site is not listed within 3 months, you may want to ask your site's status at public forum sponsored by members of the ODP community. http://www.resource-zone.com/