I am a huge NHL fan and Gretzky (and his Oiler teams) to me really saved hockey. I know hockey is down the list for the most popular sports but he really did change how it was played and got the sport more in the spot light in the 80's.
Jordan and Magic are great examples. I am not the biggest NBA guy but I do remember the days before Magic started playing the only time the NBA was on TV was on tape delay later on Friday nights. When Magic came along the sport started to be see live on the weekends and eventually week nights. Then Jordan came along and took it to levels never seen before especially off the court.
Michigan, you mention Arnie and making golf popular with the common man. Arnie's army was pretty much made up of the common man and I think he is hugely responsible for average slobs like me who love golf.
Curt Flood is another great example. He paved the way for so much for the players that they enjoy so much now.