In The Face Painter, George compares his situation with Sienna, having told her he loves her but she didn't hear, to Superman reversing the rotation of the earth. "it's like it never happened!"
In The Invitations, Jerry and Jeannie both read Superman comics.
In The Race, there are multiple references to Lois Lane. The final scene in the episode is based on one of the movies.
The Bizzaro Jerry... this one's obvious.
In The Stock Tip, George and Jerry discuss whether Superman had a great sense of humor.