Spelling Corrector in 21 lines of Python 
I have seen this one before somewhere, and what amazes me is that how you can solve problems without a hassle if you get the "trick" right. Another case I read was that Google use(at least used) two vectors(each consists of many 0s and 1s, which in turn represent whether the web page has the keyword or not) to represent web pages, and calculate the angle between the vectors to figure out the similarity(a value) between web pages.