Due in part to the episode "The Pilot" where George and Jerry pitch a sitcom to NBC "about nothing" many people are under the impression that the entire series was pitched to NBC as such. This is untrue, the series was pitched to NBC as a show about how stand up comedians come up with their material.
Superman is mentioned or a figure or a picture of him can be seen in every single episode.
Ep. 3 The Pen The only episode to not feature George and Kramer.
Ep. 14 The Marine Biologist The final monologue where George explains the golf ball was written the morning of, Jason Alexander memorized and completed the entire thing in one take live.
Ep. 15 The Susie The jingle on George's answering machine is instrumentally the theme to "The Greatest American Hero."
Ep. 19 The Yada Yada Yada yada yada is #3 on TVguide's list of the 16 greatest catchphrases.
Ep. 14 The Strongbox In The Strongbox, we find out Kramer spent a brief time in the Army. When asked for how long, he responded "That's classified."