Some of my favorite riffs are the ones based on TV theme songs. I don’t mean incidental music or random songs from a TV show, but the beginning or ending theme.
One that comes to mind is whenever you’d see someone through a windshield, particularly a woman or someone driving a white car, they’d sing “How will you make it on your own?” from the Mary Tyler Moore Show.
How many more are there?
One of my fave recurring gags is how an exterior shot of a tall building (most often an apartment of some kind) will prompt the gang to break out with the theme song from The Bob Newhart Show. It’s without lyrics, aye, but the gang vocalizing the instrumental track? Wonderful.
Yeah, humming or singing the music also counts.
One of my personal favorites is the repeated references to Hardcastle & McCormick. In one, the bots exchange lyrics for TV theme songs, Bonanza being the other.
Was just about to mention Bonanza
Anytime someone leaps off something: “MANNIX!” BA-DA-DADA!
Always liked that one! 
I think, they used to do this incidental music from My Three Sons on occasion, when you’d get an establishing shot from outside the house. At least that’s where my mind went when they’d do it, but it could have come from any TV sit-com. But, like this, for instance. I believe they did this…
Of course, I can’t remember a specific episode to confirm that.
I don’t remember the incidental music from My Three Sons popping up (that could just be my faulty memory at play here), but I remember a “My Three Skulls” riff in Eegah prompting a vocalization of the theme song.
Hot damn, I found one. I was thinking there were some of those outside the house shots in VOTG, and there it was. Yay, my brain still works!!
I like how Tom hums it and then closes it abruptly with an “Oh, God!” 
I can’t believe I beat everyone. The Gilligan’s Island Theme. So many references throughout the show. “The weather started getting rough!!!” and “A THREE HOUR TOUR!!!” are burned in my memory.
We got a movie and it’s starring Michael Landon!
The Simpsons when there’s a shot of a blue sky, or should I say “The Siiiiimpsoooooons”. I know it was mentioned in Hercules and The Captive Women and possibly others.
“What’s this ‘and the rest’ crap?”
Also, despite multiple Beverly Hillbillies references, I don’t think they ever referenced the theme song.
Also also, there’s this MST3K connection…
I love any time they reference the Greatest American Hero theme song. My brother and I used to watch that show all the time at our grandma’s house, and the first time I heard the riff with the theme song in Star Crash is the first time I first decided MST3K would be my favorite show forever.
All we need is one "y’all come back now’, but I can’t come up with one off the top of my head.
I think in either Prince Of Space or Neptune Men, they made it sound like it was being played on a koto or samisen, which just added an extra layer of funny.
In one of the Commando Cody shorts (I think) Joel laconically recites the opener to My Mother The Car, but odds are good that it was TV’s Frank “working the jaw.” 
Also, pretty sure there were at least 50 vocal nods to The NBC Mystery Movie during the Comedy Central era. 
