It's a well-known fact that One Piece manga-ka Eiichiro Oda based many of his character designs on real world celebrities. While Oda has not specifically named his influences, eagle-eyed viewers can make guesses about who might have inspired iconic characters like Trafalgar Law, Franky, and Emporio Ivankov.
DeviantArt user SATOart is one such fan. They match thirty characters with their real world doppelgangers, and created a handy chart to depict their theory.
Are these One Piece characters based on real people? Nobody can say for sure, but SATOart's work is pretty darn convincing.