Day 06 - A song that you didn't initially realise was a cover version
I'm cheating on this because I originally discovered Sloan's cover of Paul McCartney's 'Waterfalls' via a music blog's compilation of covers. Maybe this day's 'Next Day Song Challenge' should be retitled 'A song that you didn't initially realise was the last great song written by Paul McCartney'.
Sloan's great version of a great song:
And McCartney's wonderful original: