Call and response is a common form for music around the world. Typically the leader sings something, and the chorus or audience either repeats it back, or gives a set response. This is especially common in folk music, but there are also pop songs that include call-and-response style sections in the recorded versions.
To keep this from being too open-ended a list question: What is the top-charting (US Billboard) song to have a commonly known set of extra lyrics that never appeared on an official recording? Bonus points if you can explain where those lyrics came from.