City Directories are the telephone directories of yessteryear. You will be able to find your ancestor, where he lived, what business he was in, and who was in the business with him. Here is a sampling of some city directories available online.
http://distantcousin.com/Directories/
http://dir.genealogytoday.com/city_directories.html
Boston City Directories
http://dca.tufts.edu/features/bostonstreets/people/directories.html
Oshkosh Wisconsin
http://www.oshkoshpubliclibrary.org/citydirs.html
LaCrosse, Wisconsin
http://www.lacrosselibrary.org/city%20directories/
Trenton, NJ
http://trentonhistory.org/directories.htm
Buffalo, NY
http://www.buffaloresearch.com/onlinedirectories.html
Akron, OH
http://www.akronlibrary.org/internetresources/sc/CityDirectories/index.html
Allegheny County, PA
http://www.rootsweb.com/~usgenweb/pa/allegheny/citydir.htm