I think America really has most of the classic 'ghost towns' of the popular imagination. Mrs. Jack in London spent part of her childhood in Montana, and once discovered a complete ghost town while playing in the woods. She even found old jars inside one of the abandoned houses, where she and her brothers would go and play. I dread to think how dangerous it probably was!
But there are a few incredible ones in Europe too, many of which have very sad histories attached. Belchite, in southern Spain, was the site of a huge battle during the Spanish Civil War. Instead of rebuilding the town, it was left untouched as a kind of memorial. There are some incredible photos of it here:
http://www.geocities.com/lshofstra/belchite1_E.html.
There are tons in Ireland, although most are little more than piles of stones now. Many were abandoned after the Great Famine; others because of changes in agriculture over the years that meant that subsistence farmers no longer had to migrate seasonally. There's one on Mount Slievemore on Achill Island that has nearly a hundred buildings, I believe.
There are some in the UK, although some would probably qualify as more archaeological sites than ghost tows. Skara Brae on Orkney (a Scottish island) is an extraordinarily well preserved Neolithic settlement.
http://en.wikipedia.org/wiki/Skara_Brae. St. Mary's Close in Edinburgh is an almost perfectly preserved medieval street, underneath the town. (I think it was on an episode of Ghost Hunters once).
http://www.realmarykingsclose.com/I've been to a couple of small ghost towns in Cornwall and North Wales; one, the name of which escapes me, has a good tale.A couple of hundred years ago it was a fishing village, and every day the men would go out fishing while the women stayed at home. One day there was a terrible storm and the entire male population - including the boys - was wiped out at once. The women and babies who were left abandoned the village soon after, and it's just been left to rot ever since.
Great idea for a topic!