Jimi was the master.  I used to waver in my mind as to who is the greatest guitarist ever.  After watching live fotage of Jimi especially from the Montery Pop Fetival, to me he is clearly the greatest.  He and later Stevie Ray Vaughn tuned down all the strings down a half step, the E stings for instance went from E to E flat.  I'm not sure that it was only to allow them to bend the strings easier though.