Oh the wonderful things you can find in the city directory.
You can find out your ancestor's address in a given year, other adults who lived in the home with them, their occupation and the address of their workplace. You can also find out if they've recently moved out of the area and the directory will sometimes list the date of death for the husband, if the wife is still living. The directories came out annually; much better for tracking ancestors than the once a decade census. They are also fun for looking up the neighbors of your ancestors. This is another good way to find long lost relatives, as many times our ancestors lived near family.
Check Cyndi's List to find directories of the area your ancestors lived in.
images from google