Some City Directories are online in digital format, in either freely accessible databases or in commercial subscription databases. Some are only browsable, but many are searchable by surname and keyword.
Miriam Robbins Midkiff, who writes the Ancestories: Stories of My Ancestors blog, has created a website titled the Online Historical Directories Website, and has spent untold hours finding online city (and other) directories and posting links to them in an organized manner.
Here is the Home page for the Online Historical Directories site:
There are categories for Canada, Ireland, the United Kingdom and the United States. I clicked on the link for the United States:
What about California?
And on San Diego:
As you can see, the links for these Historical Directories include public libraries, websites like the San Francisco Genealogy, city directory sites like EveNDon, Google Books, and subscription sites like Ancestry.com and Footnote.com.
Of course, for subscription sites like Ancestry.com and Footnote.com, the searcher needs to be a subscriber to the service.
Other directories, like the California Voter Registrations on Ancestry.com, are also included on the list of Historical Directories.
A researcher can spend hours using this site to gather city directory data for their research targets in specific cities.
What a wonderful web site! Miriam has worked hard on this labor of love, and has a lot of work to do to keep it updated with the latest additions to online historical directories.