Some of you might remember that a while back I noted that actor/comedian Richard Belzer has appeared as Detective John Munch on six different shows: Homicide: Life on the Street, Law & Order, Law & Order: Special Victims Unit, The X-Files, The Beat and The Wire. The obvious implication of this is simple - via the transitive property, Law & Order and The X-Files exist in the same universe.
Better still, since Law & Order has crossed over with several other spin-offs (Criminal Intent, Trial by Jury, Law & Order: Los Angeles and Conviction) and The X-Files crossed over with Millennium and The Lone Gunman, that means that this TV universe contains at least 12 distinct TV shows.
And yet, Homicide has a few other crossovers to add to the mix - notably St. Elsewhere, and here's where things get interesting. See, St. Elsewhere crossed over with Cheers (no, really!). Cheers spun off Frasier and crossed over with Wings, and because of NBC's ubiquitous crossover and cameo stunts in the 90s, Frasier is linked to many other sitcoms.
So in theory, it's possible that Fox Mulder could have stopped into Boston and had a beer with Norm and Cliff, then headed to New York and butted heads with Detective Lennie Briscoe over jurisdiction.
(These are the things I spent my days thinking about when I was in college. All of this is from memory - not from research.)
Ready for me to blow your mind? In the final episode of St. Elsewhere, we were shown that the entire series had actually been a fantasy existing only in the mind of an autistic child named Tommy Westphall.
So in theory, not only do all of these shows exist in the same universe - they ALL exist in the mind of an autistic child!
One night in college I was poking around the internet, and I stumbled onto a site discussing this very phenomenon... but their take on the Tommy Westphall Universe was far more comprehensive than mine. I established connections based only on direct contact between the shows - they tend to be a bit looser, using common references between different series as enough to establish a link. Currently, the site alleges a link between 282 series!
So if you're into TV geekery and want to see just how you can get from All in the Family to CSI: NY, head on over to Tommy Westphall's Mind - A multiverse explored and drink in the TV trivia. They even have a handy chart showing how each show connects to the others.
UPDATE: Thanks to poster Nat G on Ken Levine's site, I've rediscovered another TV crossover site.