Triple G is best shifter today. Duran, Ketchel, Dempsey were great at it. If you mean switching back and forth from orthodox to southpaw, there are a few. Tyson Fury employs it.
personally I think more guys should do it for the very reason it makes it more confusing for the opponent, the trade off is its hard to learn how to do it well, one mistake in boxing can get you laid right out, so most trainers hate training it because young fighters do so many things wrong already they don't want to bother teaching a kid a whole wack of extra stuff that applies to southpaw or orthy,,,,
Cotto often switches. He is naturally left handed so he enjoys having that lead southpaw hand being stronger. Often he switches for a different angle on the jab and is quite effective.
Junior Witter used to switch like mad, at least 2-3 times every round. That guy would really wack from both sides but was inconsistent at the top level.
Prince Nassem was effective switching. Often he just jabbed from an opposite stance to create a new angle then launch one of those corkscrew jumping shots from underneath the guard.
Dimitry Pirog comes to mind. His KO of Mike Jacobs was picture perfect switch and marching forward. His transitions were excellent. Shame about the injuries as he had all the talent. Very loose, great rhythm with snapping power. Shame we couldn't get GGG vs Pirog.
Joe Calzaghe for all the flack he gets had great footwork and switched all the time. He was very nimble and had perfect timing on his jabs from both sides.