One of my favorites is from Happy Days, when Mr. & Mrs. C come home from seeing The Music Man at the movie theater. Mrs. C comments on how much little Winthrop in the film reminds her of Richie when he was a little boy. Of course, Winthrop WAS Richie when he was a little boy! 
Yesterday I saw the Drew Carey where he was dating Shirley Jones, and met her grown son, played by Donny Bonaduce.
Nobody’s mentioned the Drew Carey where Tammy Faye Bakker played Mimi’s mother. They needed a minister, Tammy Faye said “Oh, I used to know one…” long pause “but he’s not speaking to me anymore.”
In the movie Fierce Creatures, John Cleese’s character calls Jamie Lee Curtis’ character “Wanda” instead of “Willa” in one scene (actually, he says “Wanda, I mean Willa”). This a nod to the previous movie they were both in, A Fish Called Wanda
Bob Newhart, the various incarnations of his shows, and the characters on those shows have nodded so much their heads should fall off - but I love every nod.
Here is a summary. The final episode of Newhart had the Vermont Innkeeper knocked out and wake as his former sitcom character and relating this dream he had.
The link doesn’t even mention psychologist Bob going to the set of Murphy Brown to beg his secretary to return to Chicago. Nor his guest hosting SNL and waking up (again) in a skit as psychologist Bob dreaming that he’d hosted SNL.
In Hamlet, Polonius mentions that when he used to act, he played Caesar. He tells Hamlet that Brutus slew him.
It seems a safe bet that in the original productions, the actor playing Polonius really had played Caesar, and Richard Burbage would have played both Hamlet and Brutus.
Oh yeah - on an episode of Baywatch, David Hasselhoff goes on the Tonight Show. Backstage, Jay Leno tells about this great tv show where the car could talk.
Am I having some serious deja vu or are there a lot of repeats on this thread?
In Coming to America, Eddie Murphy’s character gives some money to a couple of bums (played by Don Ameche and Ralph Bellamy). One of the bums says something like, “Mortimer (or Randolph), we’re back!”. This is in reference to the fact that in Trading Places Eddie Murphy’s character bankrupts Don Ameche and Ralph Bellamy.
Edit: What cmosdes said (shows the dangers of posting to an un-refreshed thread).
Great scene – I kind of put it up there with the 3rd Rock scene for giddy awesomeness.
An episode of MAS*H had Hawkeye and Trapper switching labels on film cannisters in order to get a blue movie sent to the camp. The film they try to switch it out with is State Fair, co-starring Harry Morgan.
They’re not repeats, they’re nods to former posts. 
That’s funny right there, I don’t care who you are.
I remember an episode of “Mork & Mindy” in which Mork (Robin Williams) enters with a bunch of excited neighborhood kids and says “…And if you’re all REALLY good, I’ll take you all to see “Popeye” again!” Obviously a reference to Robert Altman’s (disasterous) film adaptation “Popeye” - starring Robin Williams.
I’m so ashamed of myself that I even remember “Mork & Mindy”, let alone actual episodes.
And going back to Boy Meets World, the kids attended John Adams High School, where
Mr. Feeny (William Daniels) taught.
One of my favorites was on the old Buck Rogers TV series from the early 1980s.
In the series, Buck is played by Gil Gerard. In one episode, for some reason they had to get a bunch of old space pilots from a retirement home and get them to fight against the enemy of the week. The retired pilots of course win and Buck congratulates their leader.
The leader tells Buck “I’ve been doing this since before you were born.”
Buck (who of course was born in the 20th century) says “You think so, huh?”
The leader responds with “Son, I know so.”
The leader was played by Buster Crabbe who, of course, played the original Buck Rogers back in the 1930s. I thought the scene was a nice nod.
On The A-Team Dirk Benedict sees a robot (that vaguely resembles a cylon) - makes a move as if to say something, then shrugs it off.
IMDb says no. "Scrubs" My Sacrificial Clam (TV Episode 2002) - IMDb
Ed Begley Jr. … Dr. Bailey
William Daniels … Dr. Douglas
Stephen Furst … Dr. Franklyn
Just thought of an obscure one. In 1979, Mary Tyler Moore starred in The Mary Tyler Moore Hour, sitcom/variety show. He character was Mary McKinnon and each show would portray her preparing and performing on the fictional Mary McKinnon Show. As a boost to faltering ratings, Dick van Dyck guest starred.
The main plot was that Mary was delayed and was unable to meet Dick van Dyke. An early scene showed Van Dyck with the writers, where they were coming up for skit ideas. Someone suggested he reprise his role as Rob Petrie. Van Dyke loved the idea, saying, “You know, Mary does look a lot like the gal who played Laura.”
There were a couple of Rob and Laura skits.
But the final topper was a great one. Mary finally made it to the rehearsal just as Van Dyke was leaving. She expresses her admiration, then said, “You know, I was up for a role in your show.” Dick says, “Is that right?” And Mary says, “Yes, but Rose Marie got the part.”
Stupid facts. Somebody remind me to go on a slapping spree on whoever is responsible for those facts.
On the new BSG, the first episode with Richard Hatch playing Tom Zareck ended up having him in a tense sit-down conversation with Captain Lee “Apollo” Adama, where Zareck was very critical of Commander Adama and the Colonial Government.
Of course, in the original BSG, Richard Hatch played Captain Apollo, son of Commander Adama.