Abandoned places are very interesting subject. I would like to do urban exploration but there's not many places to look here. Usually abandoned houses. There are only a few abandoned towns in Europe and mostly in Italy and Greece. When I visited Cretes island I found a small abandoned village with only one people living in it. It is a tourist attraction but few people know about it. Here's some information:

http://www.crete.tournet.gr/Remains_of_a_Traditional_Village-si-555-en.jsp

I have also visited catacombs in Paris, abandoned casino in Italy and abandoned mansion in Estonia.

Some links

Opacity - pictures of old asylums and mental institutions

http://www.opacity.us/locations/

Here are pictures I took from abandoned racing track called 'Keimola':

http://www.opacity.us/forum/index.php?topic=4632.0

Many former F1 drivers have driven on that track. It is just a few kilometers from my house.

Interesting places

Abandoned island of Hashima in Japan. Closed since 1974.

http://home.f01.itscom.net/spiral/hashima/hashima001.html

Prora, a holiday resort built during Hitler's regime. One of the longest buildings in the world, over 4 km long. Never finished.

http://en.wikipedia.org/wiki/Prora

Varosha, Cyprus. A city caught between two warring sides. Access not permitted, also landmines.

http://www.michaeltotten.com/archives/000985.html