George Steinbrenner was always depicted on Seinfeld, I think it helped make him an icon among people who weren't baseball fans. So I found of some of the best moments, including this cameo of George when he actually came on the show.
George Steinbrenner Cameo:
And here are some of the other moments from George Steinbrenner "The Character":
George is Crazy
I couldn't embed this last one, it is the one where Steinbrenner thinks George Costanza is dead, and goes to his parents house. And Mr. Costanza yells at Steinbrenner about Jay Buhner. Classic.