Try this on for size:
St. Elsewhere and Homicide had crossovers.
St. Elsewhere, as previously mentioned, is connected to Cheers, which we've established is connected to many other NBC sitcoms.
The X-Files crossed over with Homicide. So NOW we can link Homicide, X-Files, St. Elsewhere and the NBC sitcoms!
Homicide and Law & Order also had crossovers so we can throw that into the mix as well.
And if we *really* want to get convoluted (though this one is far more a stretch)
Shirley Schmidt on Boston Legal is invited out to dinner to a restaurant in Boston called "Melvilles"
Sound familiar? Melvilles was the restaurant above Cheers. So while not a direct "crossover" of characters it's a name drop that's obviously supposed to infer a connection to Cheers. If we accept this connection, we've obviously got to toss the Boston Legal and The Practice onto the heap of the "Cheers Universe."