Have there ever been any popular a cappella songs?

No,. but that doesn't stop collegiate a capella groups from imagining they're giving their audiences anything other than pure joy.

The closest I can think of is the Dixie Cups’ Iko Iko cover with just a drumstick sound in the background. I was also thinking of the Chordettes popular tunes, but they had sparse background accompaniment also.

Is there still a glut of those college a capella groups today? They were EVERYWHERE in the late 80s and 90s. Their pop idols were The Nylons.

They were also hugely inspired by the Manhattan Transfer (who had musical accompaniment).

Wasn't the horrid Don't Worry, Be Happy a huge a cappella hit?

R5 yes it was.

Was Billy Joel's For The Longest Time a cappella? I don't recall any instruments.

Normally I can't stand him but I thought it was a nice piece of music.

The Roaches

In 1974, a group called Prelude had a hit with an a cappella cover of Neil Young's "After the Gold Rush."

Suzanne Vega - Tom's Diner

Although there was a later dance version that was more commercially popular.

Cups by that hatchet face Anna Kendrick. She does use cups to create a rhythm but technically those are not classified as instruments.

[quote] Wasn't the horrid Don't Worry, Be Happy a huge a cappella hit?

Technically, it was not a capella. It had minimal musical accompaniment.