Combos are particularly popular in filk (the folk music of the science-fiction community), but for a lot of them, you'd need to know the source.

One of Robert A. Heinlein's short stories features a blind musician who dies heroically saving a spaceship from some problem in their atomic pile. Already fatally irradiated, he sings one last version of his most famous song, a sentimental paean to the home planet:

We pray for one last landing
On the globe that gave us birth;
Let us rest our eyes on the fleecy skies
And the cool green hills of Earth.

An amazing number of fans have written tunes for the thing, but the, um, definitive version was filked in the Midwest in the late 70s: the cad used the tune of "I'd like to teach the world to sing," and added the catchy chorus:

It's the real thing;
Earth is
What you're hoping to find
When you're reading Heinlein;
It's the real thing!

I've never heard anyone try it seriously since.