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The CBS Evening News exited in “All in the Family’s” world, as Archie used to like blowing raspberries at Walter Cronkite.
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Fred Sanford, of “Sanford and Son” appeared on “The Gong Show.” So did some kids from “What’s Happening?”
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Mrs. Naugatuck from “Maude” was addicted to the “Tony Orlando & Dawn” variety show.
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“Monday Night Football” and “Let’s Make a Deal” existed in the world of Felix Unger & Oscar Madison, as Howard Cosell & Monty Hall both appeared more than once on “The Odd Couple.”
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“Superman” existed in Lucy Ricardo’s world. “Superman” was Little Ricky’s favorite TV show, and George Reeves showed up at his birthday party on “I Love Lucy.”
Crap. That’s the one I came in here to mention.
Also, MacGyver exists in the Stargate SG-1 universe. Capt. Carter in a very early episode that it took them 15 years to “MacGyver” a device that operates the Stargate.
In The Family Guy, the list of shows that exist to them is vitually endless. Peter sent a paper clip and something else to Richard Dean Anderson to get Lois out of jail (RDA ended up poking himself in the eye with the contraption). And don’t forget the long list of shows Peter says would have to be cancelled for them to get back on the air.
Spike has watched, and commented on, ‘Passions’ and ‘Dawson’s Creek’ on Buffy.
Buffy has been referred to on ‘Grosse Point’, ‘Charmed’, and ‘Roswell’ (in fact, Roswell an an awful lot of other recent TV shows were mentioned on ‘Grosse Point’, since it takes place in the midst of the Hollywood TV industry.)
I think there was a partial Buffy reference on ‘Dawson’s creek’ too… not conclusive, but a rambling speech from Pacey about a hellmouth opening up under him and unspeakably evil monsters pouring out. 
In the CSI episode “Spark of Life” (#518), there was a television left on at a crime seen that was showing, of all things, the opening credits to Aqua Teen Hunger Force. But I guess CSI is more-or-less set in the real world, so all really TV shows (CSI franchise excepted) would be real in the show.
Roseanne also parodied Gilligan’s Island in one episode, though they never watched it. They also referenced That Girl, The Mary Tyler Moore Show and I Dream of Jeannie.
Adding to the OP, in Sports Night Casey went on the also-owned-by-ABC The View (an actual 5-or-so minute segment on the real set with the real hosts).
That reminds me of the time The Odd Couple went on Password.
Aristophanes!!!
Not only that, but when Deathshead tried to kill the Doctor, he used a Tissue Compression Eliminator gun he stole from the Master to shrink Death’s Head down to human size, and dropped the bot off in the Fantastic Four’s lap. 
There was the time Scully and Mulder were followed around by the camera crew from COPS.
In other *Buffy * references, in a Halloween episode Willow griped that she wished Buffy had chosen to wear a Xena The Warrior Princess costume. In other episode Xander wore a costume in which he pretended to be a Clint Eastwood character from a spaghetti western, or to paraphrase his own words, “I’m from Italy pretending to be from Montana.”
BtVS also references Dawson’s Creek at least once- Buffy refers to Angel as “going all Dawson” during a particularly teen soap moment of the series finale.
On Scrubs, J.D. is a fan of What’s Happening!
On House, Dr. House watches General Hospital when he’s hiding.
I think there are three shows which exist in other programs more than any other:
Gilligan’s Island
The Brady Bunch
Star Trek
All three were sixies shows that became big in the 1970s as reruns.
When the Seinfeld gang went to LA, Jerry appeared on the Tonight Show, and ran into George Wendt, star of Cheers & Corbin Bernson, star of… whatever Corbin Bernson was on. There were posters in this hallway featuring other NBC shows. Law & Order comes to mind. Also, Kramer ran into (and recognized) Fred Savage, implying The Wonder Years was familiar to them (I can’t think of anything else Savage had done to make Kramer persue him so hard.
“It’s a well known fact that Abraham Lincoln loved mayonaisse!”
The Crane men from Frasier invented a drinking game while watching Antiques Roadshow and Niles went to have something appraised when the show came to Seattle.
I thought it would have been funny in Star Trek The Motion Picture for Commander Decker to have this paraphrased conversation with Ilea (sp?) :
DECKER: The starship we are on was named Enterprise after all these other ships named Enterprise. [Points to various pictures on the wall, one at a time]. Going back, we have an aircraft carrier and a space shuttle … but the name really started with a fake spaceship from a cheesy 1960’s television show calleed “Star Trek.” [Zoom in on picture of Enterprise from Star Trek TOS].
Magnum, P.I.:
Simon & Simon
Murder, She Wrote
There are a few others, but I can’t recall which ones they are right now.
Yes, I already mentioned the connection that way, with Spike watching the show in his lair or something of that sort. Forgot that Buffy mentioned it too.
Just caught the last couple of minutes of a L&O episode in which DA Dianne Weist was going to be interviewed by Stone Phillips, so Dateline NBC exists in the L&O-verse, and by extension the Homicide-verse.
Also from Golden Girls…
St Elsewhere, Rose’s blind sister mentions that it’s her favorite program.
The Cosby Show, the girls decide to pay for a neighbors funeral, even though they all hate her. But the funeral can’t be on thursday because Cosby is on.
Another World (a soap), Blanche insists on watching it while the girls are all home sick.
They also read the comics, Blanche reads Marmaduke and Apartment 3G every day.
Russell, who knows WAY to much about GG