Free Illinois City Directories are available at Internet Archive.
City directories contain an alphabetized list of residents with their address, spouse’s name, profession, and often a reverse lookup by address. They often contain historical information about a locale and they’ll usually contain a separate business directory.
Individual Illinois City Directories:
1896 Belvidere City Directory (files must be downloaded)
1910 Belvidere City Directory (files must be downloaded)
1908 Paris City Directory (alternate source)
1880 Rockford City Directory (files must be downloaded)
1909 Shelbyville City Directory (alternate source)
1857 Springfield City Directory (Sangamon County)
Albany City Directories (intermittent from 1866 to 1905)
Champaign-Urbana City Directories (intermittent from 1878-1940)
Chicago City Directories (1844, 1911)