I want to keep this as a seperate thread from the on-off episodes thread.
What are some instances where a character – or entire cast – from one TV show guest starred on another? For instance, on The Single Guy, the main character became friends with Ross (from Friends). (They both suspected the other of being gay.)
The Law and Order shows would cross over. I remember seeing Jerry Orback on SVU and there were other characters as well.
The Critic Jay Sherman on the Simpsons.
Fran Drescher & Ray Romano went to the same high school, and he appeared on her show.
3rd Rock from the Sun had an episode with Harry (French Stewart) being the head of Saturday Night Live (In one of the best guest shots ever, they also had Dennis Rodman starring as a fellow space alien)
Early in the series, Ray Romano and sometimes his mother made appearances on King of Queens. Memories iffy as to how many times but at least one episode centered around Ray and Doug spending the day together playing golf.
In one episode of St. Elsewhere, some of the doctors went to the bar from Cheers.
Several of the characters from Cheers guest starred on some episodes of Frasier
The Simpsons also has had guest stars from Cheers and The X-Files (and probably others shows, too).
There was an epispode of Futurama featuring several characters from the original Star Trek.
There were Friends/Mad About You/Seinfeld connections which implied a shared universe. (and by way of the OP, this would include the Single Guy)
Ray Romano guest stars on King of Queens if I remember correctly.
Magnum PI and Murder She Wrote crossed over.
*Cheers and Wings (and naturally, Frasier) shared a universe.
Well, duh. Ursula the waitress on Mad About You had an identical twin, Phoebe on Friends.
Mr. Carlin (From the Bob Newhart Show) played a screwed up psychology patient on Newhart. (“I had to undo years of damage done to him by some quack in Chicago.”)
Lawyers from The Practice visited the offices of Cage and Fish (Ally McBeal).
Ursula appeared in several episodes of Friends as well. Jaime from Mad About You was on Friends at least once, she visited Central Perk and mistook Phoebe for Ursula.
The character of John Munch from Homicide: Life on the Streets not only guested on the original Law and Order, but crossed over completely to the Law and Order: SVU series. He has also been in an episode of The Simpsons (voice only, duh) , The X-Files, The Beat, the short-lived Law and Order: Trial by Jury, and Arrested Development.
He’s the most crossed-over character in TV history.
And Paul sublet his apartment to Kramer.
Didn’t the good folks from L&O once visit Baltimore?
Someone with a lot of time on their hands created this site a while ago.
Crossover and Spin Off Alphabetical List
They appear to be keeping it up to date.
Jim
One Simpsons episode had a cameo with Bender, the robot from Futurama.
Yes. There were several L&O/Homicide crossovers.
But I’ll be a spoilsport and link to the definitive list.
Missed by 4 minutes. 
Frasier appeared to give a seminar on an episode of Wings.
JIll Hennessy and Jerry O’Connell of Crossing Jordan made several appearances on Las Vegas.
They’re missing Kramer’s turn as Murphy Brown’s secretary.
There have been cross overs with ER and something… police drama cant remember what it was called. The epside where Susans sister Chloe goes missing and she has the child, and theres a cross over into the police drama.
Also more recently, spolier in case people dont want to know/may not have been shown yet
Theres a crossover between CSI and Cold Case where one of the CSI’s DNA turns up on an old Case
The show is called Third Watch.