That's the one I was trying to remember...the Beatles "Twist and Shout" is a direct ripoff of "La Bamba"

This is a little off topic but another thing I was gonna mention is take a look at the usenet newsgroups for bootlegs. They're constantly posting demo's and outtakes from famous songs.
I was just listening to a really early version of Stairway to Heaven and it has a completely different chorus with different chords. Another one with an early version of Immigrant Song by Led Zepplin with the vocals sung in a different timing at the beginning. A home demo of Pete Townsend practicing guitar.
An early instrumental version of Us and Them by Pink Floyd. Etc etc..

It kinda gives you an idea how the ideas progressed along the way as they were writing the songs and how just a few simple chord or tempo changes can make or break a song.
The home demos also show you how the guitar 'greats' sound when they're just at home by themselves with a tape deck...kinda weird to hear them making mistakes and screwing up just like everybody else.