Directories can be very helpful genealogy resources. They may include a little or a lot of information. Some details you may find in directories include the spouse's name, address, occupation and employer. If you are having difficulty finding an ancestor in the census, the directory can help you pinpoint their location. Some directories are organized by street in addition to name, which allows you to find neighbors and possibly other family members living nearby.
That is just for city directories. There are also alumni, business and organization directories which may yield even more information.
Online Historical Directories is a nice free genealogy resource if you are seeking out directories. The site owner is attempting to compile a list of online directories beginning with the United States and Canada and then moving on to other countries. There are already some directories from Ireland and the United Kingdom.
Not all directories are free; some require a subscription to access them. The webmaster has taken the time to mark each directory as $, Free or Free*. Directories marked with the dollar sign require subscription or payment to access. Many of these are with Footnote and Ancestry. Both of these sites offer free trials so you can use the trial to access if desired.
The ones marked free take you directly to the directory with no other requirements. The ones marked free with an asterik are free, but require registration or a library card to access. I like the fact these are marked differently from the other free ones as
This free genealogy resource is very much a work in progress. Some states may only have a small number of directories available while others may have more.
Be sure to check out the articles and resources links as well. The articles page gives you information on how to use directories and why you may need them for genealogy research. There are also site-specific tutorials on using directories. The resources page gives you some more options for finding directories.