Detroit is the Holy Grail of urban exploration locations is Detroit, Michigan. Due to many different political, economical and cultural problems (I have my own opinions on Detroit's decline, but I'll keep them to myself) Detroit went from being a beautiful city full of culture and prosperity, a veritable boom town, to having nearly a quarter of it's peak population numbers all in the span of a century. It has left many incredible buildings and locations abandoned and open for people like myself who enjoy urban exploration. Here is a website that I have found detailing many of Detroit's most interesting abandoned landmarks in photographs. Enjoy.
http://www.detroiturbex.com/content/index.html
http://www.detroiturbex.com/content/index.html