City Methodist Church, Gary, IN

I drove through Gary, IN on my cross-continent move from Buffalo, NY to Victoria, BC and didn't get to spend nearly as much time there as I would have liked to. But I was there long enough to see that, as I'd read, it really is a half-ghost town. 

From Wikipedia: Gary "was founded in 1906 by the United States Steel Corporation as the home for its new plant," but "in the 1960s, like many other American urban centers reliant on one particular industry, Gary entered a spiral of decline [...] brought on by the growing overseas competitiveness in the steel industry, which had caused U.S. Steel to lay off many workers from the Gary area. [...] Owing to white flight, economic distress, and skyrocketing crime, many middle-class and affluent residents moved to other cities in the metro area such as Chicago." The population is now around 80,000, and "as of 2013, the Gary Department of Redevelopment has estimated that one-third of all homes in the city are unoccupied and/or abandoned."

And it's not just homes; this massive church has been empty and unused for almost 40 years.

I stopped by this one specifically after seeing it mentioned online. If you're interested, here's an article detailing its whole history; the article also has a lot of photos, past and present. It sounds like the building was in okay shape until a 1997 fire, which destroyed part of the roof.

When I was there (August of this year) you could walk right in, no fences or boards on the doorway, and I would have loved to explore it, but since I didn't have the time (plus it was kind of creepy), I contented myself with just walking in a little way.

And it's actually a good thing I didn't spend too long or go in any farther, because this website (which also has an awesome virtual tour) says you need a permit from the city to explore and photograph abandoned buildings. Luckily I didn't get in trouble for any of the photos I took.

This sign was outside, but I'm not sure why since it's got the wrong name...

At one point the city had plans to turn part of the building into a ruins garden, which would have been awesome, but there wasn't enough funding. It does get used, though, by photographers and also in movies (including, recently, Transformers 3), so unlike abandoned buildings that just get boarded up and left to rot, it is still serving a purpose.