The sources of these links are varied. Some of these links were obtained from DP, sitepoint, others, some from google, some from the confirmation emails that directories send out where they advertise other directories, some from friends, some from websites' "links" pages etc...I have submitted sites to all these directories so I know that at one time they were all free and hopefully they still are. As I submit, the list grows.
I bought a software program that comes with about 4000 sites, about half are free and you can add more. It is called submit site, it is semi automated. You need to enter information and confirm, works well I do a few every day in a few minutes. There is also site that is a quasi directory quasi article directory site where you can get multiple links. You add articles or even short sentences, even stuff from your blog and you get a link. The rank of your links increases based on your reviews of the stuff that is submitted and based on what you submit. They claim you can also get the ad revenue if your stuff gets a lot of views. You also get link rank credit for referrals. I am not associated with them I opened an account and if sign up through this link I get credits. It is all free. http://arnold2001.qassia.com
Here is a simple SEO Web Directory www.Directory-Stop.com add it to your list.. if you donate he adds your link in 24 hrs i sent him $1 and my url was added 10 mins after that.