Kinda obvious one
Why Dont You Get a Job by The Offspring and Obla-di Obla Da (sp?) by the Beatles are basically the same song
And I think Alice Cooper's Give it up sounds like Motley Crue's Looks that kill (though, my dad would say the remixed Live Wire from their Decade of Decades album)
Also, maybe I'm the only person in the world who would notice this, but in The kids Arent Alright by the offspring, the first few notes in the riff was taken pretty much note for note from the very short guitar solo (by short, I mean the 4 seconds about halfway through the song with just guitar) ramones' pet semetary.