Baltimore City Directories
There is a very good collection of city directories going back to 1796. Most are online at the Maryland Archives, the U of Maryland, or Ancestry.com. These are very useful, since they are mostly searchable by a PDF viewer so, for example, you can search for anyone else at the same address as your ancestor.
One of the most important issues is the 1887 directory which provided the conversion between old addresses and the new scheme which was instituted that year and continues until the present. It is not online at any of the above websites. Although the UofM website lists the 1888 version as "giving both the old and new numbers", it does not. Only the 1887 issue provides this information. A PDF copy (of the address conversion part) is available here (62MB).
Besides providing an alphabetical listing of residents, many of these directories also have the following useful sections:
Other helpers for Baltimore area records:
Updated 26 May 2017