Almost anything you build on the web has already been tried in one form or another. This should not deter you. Antecedents existed for Google, Facebook, Groupon, and almost every other tech startup that has succeeded since the dot-com bubble. Entrepreneurs should always ask themselves “why will I succeed where others failed?” If the answer is simply “I’m doing it right” or “I’m smarter,” you are probably underestimating your antecedents.
(Full Post: http://cdixon.org/2010/11/07/timing-your-startup/ )