National City, IL. It's just a fabulous abandoned town. Go South on highway 3, just a bit south of the McKinley bridge.
East St. Louis IL -- once had a population of 80,000 now it's just hovering around 30,000.
Cahokia Mounds - Doesn't get more abandoned than that. It was once a great city, but now all we have is memories of a pre-Columbian settlement whose ways we never studied. They moved out before the first wave of illegal immigration.