I'll name that song in 5 shows

Music follows a set of rules; there are 7 notes in a diatonic scale, some chord sequences work better than others and these rules make  a collection of sounds pleasant for the human ear. They are also extremely limiting and so musical plagiarism is everywhere.

It's what we like, though.  If I was going to write a pop song I would reference a nursery rhyme and take people back to their childhoods, where the sun shone all summer, everyone was their friend and they were never tired.  The beginning of Somebody I used to Know by Goyte does this brilliantly.

http://www.youtube.com/watch?v=8UVNT4wvIGY

Musicals often add in themes from other bits of music.  They are usually themes from popular culture at the time that help the audience identify with the story that is being told.  Avenue Q, is one of my favourite musicals because it doesn't pretend to be anything other than a reference to Sesame Street and Children's TV shows.

Little Shop of Horrors has the Goodies theme tune in it.  Every night this week I've come home and watched a clip from the Goodies on YouTube.  Oh, how I loved the Goodies but now it makes me laugh for all the wrong reasons.  I wonder if Graeme Garden and Co watch Ecky Thump back and cringe at the racist, politically incorrect jokes that were acceptable in the Seventies.

The Goodies just had the best songs as well.  I loved, "Everybody Loves String.", "The Funky Gibbon" and "Father Christmas Do not Touch me."
http://www.youtube.com/watch?v=hC3AphnJLbE

This song takes me right back to my school days.  Those glorious days in the 80s where teachers were allowed to be human and no one complained.  Our school choir was a Staff and Pupil choir.  The music teacher bribed his mates (with beer I believe) to sing Tenor and Bass.  A group of rugby playing maths teachers taught the whole choir how to sing "Sir Jasper", which we thoroughly enjoyed, especially when we got to breathlessly moan, "Oh..........Oh............ Oh,"

At the beginning of  The Meek Shall Inherit (in Little Shop of Horrors) a musical reference is made to a   TV crime show from the 1970's.  It's not Miss Marple, Murder She Wrote or Poirot.  I can't quite work it out at the moment but there are still two shows to go, so maybe it will come to me.

http://www.youtube.com/watch?v=vhCo6PUVf3k&feature=relmfu