Dmoz is the largest directory on the Internet and believed to offer considerable weight in its backlinks. Dmoz is entirely free and run by editors who follow set guidelines to decide whether or not to list a site in the directory.
One of the problems of Dmoz is that it can take many months for sites to show up in the directory at all. There is also a recent trend of webmasters signing up as editors and then exploiting the section that they are moderating.