Chicagoans: The next time you give directions to your friends, you can thank this guy - Edward Brennan - for making it a little bit easier.
Before 1909, addresses were generally designated by the branches of the Chicago River. Although it was simple, since the river doesn't flow in a straight line, it would easily confuse people. This got even more confusing as Chicago expanded and annexed the surrounding land. Soon after the turn of the century, Brennan, who was a building superintendent, decided it was time for a change. Finally, after 8 years of planning with the city, the new system was implimented in 1909 (and in the Central Business District in 1911).
Over 100 years after the switch, many residential buildings still display their old addresses (along with their current ones, of course). The majority are in stained glass above the door, but some are set in stone. The largest density of these old addresses is in Ukrainian Village and Wicker Park. Check out this map to see where all of them that are known are located. If you know of any more, please leave a comment!
Some of the streets have also been renamed. Keep in mind that some streets that are currently one name all the way east to west and north to south were sometimes over a dozen different names depending on your location. This 1904 map shows the layout of the city.
You can learn more about the old address system at Forgotten Chicago.