In Japan, it's the number 4. This is because one way of saying the word for "4" is "shi". Written with a different kanji, the pictogram writing symbols that the Japanese have adopted and adapted from China, the word "shi" means death. So there are many buildings in Japan without a 4th floor. This link says that 9 is also considered unlucky, though I didn't hear anyone say that while I was there.http://japanese.about.com/blqow7.htm
I either already covered 4, or have one in the queue to post.