Now that I read it again, maybe he is looking for one-off episodes, but that shouldn’t have been very hard to think of tons of them as evidenced by this thread. I’m thoroughly confused.
Some of the detectives on the old Quinn Martin shows of the Seventies did crossovers. Barnaby Jones (Buddy Ebsen) first appeared on an episode on William Conrad’s Cannon, and there was one week where an episode of Cannon ended in a cliffhanger that was resolved on Barnaby Jones.
There was an episode of E.R. in which Sherry Stringfield’s junkie sister was missing in New York City, and she sought help from police officer “Bosco” Boscarelli from Third Watch.
On the short-lived Seventies legal sitcom The Associates, John Houseman appeared as Professor Charles Kingsfield from The Paper Chase. Houseman was supposed to be an old enemy of corporate lawyer Wilfrid Hyde-White, and they traded insulting witticisms (I remember Hyde-White sneering “Those who can, do. Those who can’t, teach.”
Magnum PI and Simon & Simon
X-Files and COPS
X-Files and Picket Fences
X-Files and The Simpsons
Family Matters and Full House
I never saw Picket Fences, but I’d like to at least the crossover episode.
Dallas and The Simpsons
They could have squeezed him into the one with the magician in the coffin. Just a cameo, where he’s explaining to someone, “Not tricks, illusions. A trick is something a whore does for money.”
NCIS’s Abby showed up in a couple of episodes NCIS: Los Angeles. And NCIS Director Leon Vance has been there occasionally throughout its whole run.
Back in the early 90s there was the same tornado that blew threw 3-4 sitcoms that were in a row (on the same channel). I don’t remember all the shows that were involved, but one of them was Roseanne. It was dubbed something like “Tornado Night!”
And on November 7, 1997 the cat from Sabrina the Teenage Witch was observed running through the 3 shows following it while Sabrina was trying to catch him (Boy Meets World, Teen Angel, and You Wish) at some point.
The Pretender and the Profiler had one where they appeared on each other’s shows.
I think the power went out in New York for an entire Must See TV one night, too.
Community had a crossover with Cougar Town. Sort of.
It’s also revealed at some point that Paul from Mad About You and Kramer from Seinfeld used to be either roommates or neighbors.
Ally McBeal and The Practice had a two-parter that started on one and ended on the other. Especially special since the two were on different networks, and one was a comedy and the other a drama.
I’m thinking that either Kramer was subletting Paul’s old apartment or Paul was subletting Kramer’s old apartment.
Yeah, you’re right. It was something like that. In my head, it happened the same night as the backwards episode of Seinfeld, but I can’t be troubled to look it up right now.
I would like to go on record and say that the Magnum P.I. and Simon & Simon crossover is one of the greatest events in television history.
(At least it was when I was 14.) 
Picket Fences and X-Files never did a crossover episode. There were some plans to do one but CBS decided they really didn’t need to be promoting another network’s television program.
Crossing Jordan did at least two crossover episodes with Las Vegas.
Supposedly Jill Hennessey (who played Jordan), met Josh Duhamel of Las Vegas at some network event, and wanted to work with him.
I just watched several of the Law & Order/Homicide crossover episodes on DVD. The relationship between Briscoe and Munch was a thing of beauty – they really deserved their own spinoff show.
Sons of Anarchy and the Shield are kinda/sorta the same universe. Mostly it’s just references, like shared actors, but not characters, but apparently there are references in Sons. The One-Niners are a street gang mentioned in both.
Nope. Same night as “The Virgin.”
And according to IMDb, Kramer was sub-letting Paul’s old bachelor pad.
Yes, as you say the CSI writers wrote an episode of Two and a Half Men (a couple of CSI actors guest starred); then the Two and a Half Men writers did an episode of CSI (where an unpopular female TV star was the first murder.)
There was also an episode of “Green Acres” where Lisa and Oliver are reminiscing about how they met during World War II. When Oliver tells the control tower back in England that he has to parachute out his his plane, the flight controller advises him that if he ends up in Stalag 13 to look up a chap named Hogan.
Werner Klemperer made an appearance during a Bat Climb where he referred to as Colonel.
There is a Desi Arnaz Lucille Ball Comedy Hour episode where they sublet their house to the Danny Thomas tv family. Ends up in court where the judge is Gale Gordon, who was almost Fred Mertz and would become Mr Mooney.