Actor Allusion: In one Halloween episode, Frank dresses up like Frankenstein's monster (with Marie as his bride). At first this just seems like a quick gag on Frank's name until you remember Peter Boyle played the monster in the hysterically funny parody Young Frankenstein.
Ray's crossover on The Nanny at his, Fran Fine's, and Val Toriello's high school reunion. Truth in Television, as actors Ray Romano and Fran Drescher really did attend Hillcrest High School in Queens, NYC in the same class.
Patricia Heaton's new show The Middle has her meet her son's new teacher who is played by Doris Roberts. Pretty much their roles are verbally reversed: "Marie" accusing "Debra" of being a Beloved Smother (which ironically is absolutely not true in either show).
The third season premire of The Middle features a flashback in which we see Heaton's character's honeymoon/camping trip interrupted by a hiker played by Romano. At one point his character tells her "In an alternate reality, you and I could have been happy together."
One episode had J.G. Hertzler play an old book author who, while rambling on at a bookstore, causes Ray to fall asleep. He happens to be missing an eye...
They only had occasional guest appearances, but Ray Romano's father Albert and Phil Rosenthal's father Max appeared sporadically as friends of Frank named "Max and Albert".
Comedian Andy Kindler played one of Ray's best friends, Andy.
Kevin James originally played a guest character named Kevin, until he got the spinoff King of Queens, after which his character on "Raymond" was retconned into being Doug Heffernan, just like on "Queens."
Friday Night Death Slot: The show originally aired on Friday nights, but after the first season, it was moved to Monday nights, where it stayed for the remainder of its run.
Uncle Leo appears on this show a few times as well.
Chris Elliot as Amy's brother
Fred Willard as Amy's father
The Other Darrin: Amy's brother was initially played by Paul Reubens. When the character returned, he had a different name and actor, but was clearly meant to be the same comic book store owner that grew attached to Ray. Off screen, Reubens was arrested on public indecency charges, and although he managed to stay out of jail, Ray Romano didn't like the idea of bringing him back and requested that the character be recast. Oddly, not only was the character recast, but renamed (Russell to Peter)
Nemo was originally played by Joseph V. Perry, but the actor died in February 2000; his character was subsequently played by Robert Ruth in "What Good Are You?" and "Stefania Arrives".
Post Script Season: The series was originally intended to be only eight seasons long, but Ray Romano and Phil Rosenthal decided to give one last season to the fans. As a result, the final season was a bit shorter than the previous ones.