Hillary Clinton (not played by herself) was seen in the episode The Weaker Sex. Hillary Clinton's double was president of the United States in 1995 at the same time that Bill Clinton was president on Earth Prime.
Marin Luther King, Jr.: As of 1996, his double is still living in Time Again and World as a political prisoner in Alcatraz prison on a world where the United States declared martial law and the United States constitution has been outlawed.
Newt Gingrich: In Prophets and Loss, a person known as "The Oracle" had the highest authority in hte United States. Upon seeing his picture in the newspaper, Quinn said, "That guy looks like Newt Gingrich."
Janis Joplin: In Stoker, she is either still alive or a vampire, whichever the case, Quinn mentioned her (she didn't die of alcohol poisoning as she did in our world)
George Bush: In The Good, The Bad and the Wealthy, George Bush was President of Texas (according to Quinn mentioning a portrait).
Ronald Reagan: In Greatfellas, he was seen in a billboard regarding an ad for his campaign for governor for California (it said, "you liked him as president").
Jerry Seinfeld: In the fifth season episode "The Java Jive", the show is mentioned when referenceing a character's usual tidiness.