Fake Nationality / Fake Brit: Daniel Davis as Niles, who is from Arkansas. He is good at it though, to the point where Charles Shaughnessy (Maxwell Sheffield), an actual Brit, was told by some critics that he should take lessons from Davis in order to get his "fake" accent right.
Lampshaded in a late episode, where Niles has a bit part in Gracie's play and does very badly. Citing a review, he asks Max, "Do you think my accent sounded fake?"
Hey, It's That Guy!: Multiple times, one example being the episode "Kissing Cousins" where Fran's date( who she later finds out is her cousin) is Jon Stewart.
James Marsden was Eddie, Maggie's first boyfriend during the first season.
Chester the Pomeranian was Fran Drescher's real-life dog, which is why he was always so excited around her.
Fran Drescher's real life parents Morty and Sylvia appeared on the show in several roles, later on as Uncle Stanley and Aunt Rose.
What Could Have Been: Originally, there were just two Sheffield children: Brighton and Gracie. The executives suggested that a shy teenaged daughter would serve as an amusing contrast with Fran, which eventually became Maggie.
You Look Familiar: Steve Lawrence, who played Fran's (mostly) unseen father Morty, had appeared as himself with wife Eydie Gorme earlier in the show's run.
A couple of the extras who play nameless one-shot minor characters like a doctor reappear later as a security guard or priest.
Not just extras, several celebrities had multiple appearances, once in character and once as themselves. Like Steve Lawrence, it happened with Coolio, Whoopi Goldberg and Rosie O'Donnell as well.
The girl who played Fran's Generation Xerox cousin Tiffany in a first-season episode played a completely unrelated high school girl named Francine in a fourth-season episode, with both characters being written as "Just like Fran only Brighton's age."