Those with Cook County, Illinois, ancestors will really get some use out of this website. A Look at Cook contains census enumeration maps and other ward maps for the City of Chicago from the 1870 through 1930 time period. The site is free to use and very helpful in determining what enumeration district your ancestor should have been in for a particular census.
I've made extensive use of it while working on my wife's Chicago area families between 1870 and 1935.
The website is easy to remember: www.alookatcook.com
Michael