Oh, and I just thought of this, on Due South, they would occasionally mention that RCMP Sergeant Buck Frobisher bore an uncanny resemblance to “world renowned Canadian actor Leslie Nielson” (Buck was played by Leslie Nielson, of course.)
“The Nanny” also had an episode where CC babbled about an episode of “Seinfeld” she just saw where Elaine was about “24 months pregnant” and how they tried to hide it–the punchline being that Lauren Lane (who played CC) was wearing a large coat and carrying a huge purse, and picked up a huge plant and said “this needs watering” using all those props to hide her own pregnancy.
And vice versa… Louise once asked Herman if he thought her voice really sounded like Lisa Simpson. (obviously, both of these qualify as in-jokes, especially since they both involve Yeardley Smith.)