If you have old addresses for ancestors in Chicago in the 1800's or early 1900's, and you want to see where those streets are today here are two great free websites.
1880 Chicago map:http://www.davidrumsey.com/detail?id=1-1-30505-1140032&n...
This can be enlarged so that every street name can be read. There is also an index on the bottom. In 1880 Chicago only went as far south as 39th Street, but by November of 1890, voters had approved so many annexations that Chicago extended well south of 103rd Street.
Chicago street name changes:http://www.chsmedia.org/househistory/nameChanges/start.pdf
This can be a little hard to understand what name was old and what section of it was changed to what name, but this is the only place online where I have found a list of Chicago street name changes going back many many decades.