Michael Phelps is the best swimmer of this generation, if not of all time. His achievements at the 2008 Olympic Games are well documented and will be difficult to match ever again.
Yet, from a pure swimming perspective, Phelps has been part of a revolution regarding the way freestyle is being swum. This was clearly demonstrated in the 200 m Freestyle at the Beijing Olympics where Phelps used a hybrid stroke that incorporated hip driven and shoulder driven freestyle.
The old way of 3 strokes and breathe has also been done away with and the top swimmers in the world mostly favour a 2 stroke and breathe technique.