plenTpak52
Pinball Wizard
Stairway to Heaven, the Lemon Song, Rock & Roll are the first three that come to mind.
Stairway was taken from the song Taurus by a band called Spirit. You can find it on iTunes if you want. The Lemon Song i am not really sure about, but I've just heard some of my friends who are big Zeppelin fans admit to it (they then say the song is shit anyway). Rock & Roll, although it is a classic blues riff, is a complete rip off of Long Live Rock by The Who. Not only is it the same chords but its the same idea all together. They also sort of took their name and artwork. The Who's Keith Moon and John Entwhistle were going to break off from the band because they were upset with how Pete was starting to take control of the band. So they went into the studio with Page and i think Jeff Beck, wanting to form a band with them. Keith said the whole idea would go down "like a lead balloon", which was where John got the idea for the name Lead Zeppelin (later changed to Led). Then John made the pic of the Zeppelin going down in flames. When they decided not to leave the Who, Page asked if he could use the name and artwork.