There are a great deal of directories out there. How do you know which ones are worth submitting your website to and which ones are not worth your time. If your goal is to increase the number of links back to your website, and possibly your PageRank, you should look for SEO friendly directories. SEO friendly directories are those directories that link directly to your website without using a rel=nofollow attribute on the anchor tag. The best list of SEO friendly directories I have ever found is at Info Vilesilencer. The directories are listed alphabetically along with PageRank of the directory home page. You can also download a copy of the list as a spreadsheet, so you can use the list offline. It is a great resource to help build some links to your website that are SEO friendly.