The biggest directory project is the open directory project. If you didn't know about it...well, now you do. Its also called ODP or the "Google directory." It is supposed to be hard to get approval but if you follow the basic steps you should never have a problem with the ODP (or any other directory for that matter). Other big directories include the Yahoo Directory; we will go into the hundreds of others later.
Most directories have human editors and frown severely on multiple submissions, however if you have a big site with multiple categories, you can place your subcategories as sub domains and submit in different sections of the directory. For example, if you have a web site that has one category on Windows programming (software) and another on networking (hardware), if you divide both into different sub domains, you can put one sub domain under "software, Windows programming" and the other under "hardware, networking." It will not be considered spam since the two URLs cover separate topics. Note however that for branding purposes it is always best to have one web site which covers one topic, and in case you run into an overly strict "volunteer" editor (most of them are overworked and not paid anyway), it is sometimes better to submit just one URL to a directory from your site.
Directories want to offer a service. If they are good at listing, they get traffic; if they are bad, they don't. They need relevant listings. This is why some people find it hard to get listed; a lot of sites try to get listed on unpaid directories without being relevant. This will definitely make directory submission hard.
Your directory submission should be done when your website is fully functional and ready for viewing. Some advise that it should be done once your domain name is registered, but then there will be nothing for the humans to review. You must have finished building the basics of your site before you submit your URL for approval.
Web Directories 
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