Accusations of plagiarism against classic rock acts aren't uncommon, but complaints against Led Zeppelin seem to occur far more frequently than for their contemporaries. While many artists in popular music have been accused of plagiarism or have had lawsuits filed against them, accusations against Led Zeppelin seem to have stuck to them the way they haven't to most others. Wikipedia actually has a page tactfully titled "List of Led Zeppelin songs written or inspired by others" although the list also consists of songs the band did properly credit. I couldn't find a comparable Wikipedia page for other classic rock artists.
One of the most damning cases of plagiarism against the band involved the song Dazed and Confused. In 2010, folk singer Jake Holmes sued Led Zeppelin for plagiarizing the song which was released on his debut album in 1967. At that time, Jimmy Page was a member of the Yardbirds. Holmes had opened for the Yardbirds at a Greenwich Village performance in August 1967. According to Holmes:
"That was the infamous moment of my life when Dazed and Confused fell into the loving arms and hands of Jimmy Page."
After the Yardbirds broke up, Page brought the song to his new band Led Zeppelin. The song appeared on the group's 1969 debut album credited solely to Jimmy Page. While the resemblence to the Holmes song has been discussed for many years, it's unknown why he waited so long to file a lawsuit. In 1990, Page was asked about Holmes:
"I haven't heard Jake Holmes so I don't know what it's all about anyway. Usually my riffs are pretty damn original."
In 2012, a settlement was reached in the case and Holmes is now credited as a writer.
Whole Lotta Love from 1969 is lyrically very similar to the 1962 song You Need Love by Muddy Waters. Songwriter Willie Dixon sued and received a settlement. Led Zeppelin also borrowed from the Dixon track Bring It on Home by Sonny Boy Williamson. According to Page:
"The thing with "Bring It On Home," there's only a tiny bit taken from Sonny Boy Williamson's version and we threw that in as a tribute to him. People say, "Oh, 'Bring It On Home' is stolen." Well, there's only a little bit in the song that relates to anything that had gone before it, just the end."
The problem was Dixon was never given any credit on the song. Howlin' Wolf (Chester Burnett) didn't initially receive credit on the Lemon Song despite similarities to his song Killing Floor. Anne Bredon's lack of writing credits for Babe I'm Gonna Leave You, also from Led Zeppelin's 1969 debut, seems to have been an error rather than theft. The band mistakenly thought it was a traditional song with no credited songwriter when they covered it. Bredon was unaware until the 1980's that Led Zeppelin had covered her song.
Scottish folk singer Bert Jansch never sued for Black Mountain Side (similar to Down by Blackwaterside) and Bron-Y-Aur Stomp (similar to The Waggoner's Lad). While he never sued, Jansch wasn't happy about it. He told an interviewer in 2007:
“The thing I’ve noticed about Jimmy [Page] whenever we meet is that he can’t look me in the eye...Well, he ripped me off, didn’t he? Or let’s just say he learned from me. I wouldn’t want to sound impolite.”
While there haven't been any lawsuits, many people noticed similarities between Since I’ve Been Loving You and the 1968 track Never by the indie band Moby Grape. In Led Zeppelin Dazed and Confused: The Stories Behind Every Song, author Chris Welch said:
“Some have claimed Moby Grape’s ‘Never’ as the inspiration for the tune and certainly the band were one of [Robert] Plant’s favourites.”
Led Zeppelin was also sued for plagiarizing Taurus by Spirit but they won in court. That lawsuit involved some of the instrumental portions of Stairway to Heaven. Lawyers for Spirit have filed an appeal in the case.
"Led Zeppelin not only opened for Spirit in 1968, played several shows with them, covered a Spirit song, and owned several Spirit albums, but Mr. Page also extensively praised Spirit in interviews before and after he created “Stairway to Heaven” in 1971. Despite Led Zeppelin’s denials, the jury was unequivocally clear that Led Zeppelin had access to “Taurus,” one of the key elements in a copyright infringement case.”"
Led Zeppelin have admitted that they did take from the works on others. And these quotes have been used against them in court. Jimmy Page told one interviewer:
"And Robert [Plant] was supposed to change the lyrics, and he didn’t always do that– which is what brought on most of the grief. They couldn’t get us on the guitar parts of the music, but they nailed us on the lyrics."
Another time Robert Plant told an interviewer:
"I think when Willie Dixon turned on the radio in Chicago twenty years after he wrote his blues, he thought, ‘That’s my song [Whole Lotta Love].’...When we ripped it off, I said to Jimmy, ‘Hey, that’s not our song.’ And he said, ‘Shut up and keep walking.'”
A defense could be made of Led Zeppelin in all of this. No artist is actually original. Everyone is building on the work of everyone else. Or as Pete Seeger put it:
"Plagiarism is the root of all culture."
Woody Guthrie would have agreed. He talked about making modifications to the works of others.
“The words are the important thing. Don’t worry about tunes. Take a tune, sing high when they sing low, sing fast when they sing slow, and you’ve got a new tune.”
Many other greats are in agreement as well.
“All ideas are secondhand, consciously and unconsciously drawn from a million outside sources. We are constantly littering our literature with disconnected sentences borrowed from books at some unremembered time and now imagined to be our own.” -- Mark Twain
“Good artists copy, great artists steal.” -- attributed to Pablo Picasso
"Originality is undetected plagiarism." -- William Ralph Inge
"The good poet welds his theft into a whole of feeling which is unique." -- T.S. Eliot
In Plagiarism in Dylan, Or a Cultural Collage? Jon Pareles of the New York Times wrote:
“Ideas aren’t meant to be carved in stone and left inviolate; they’re meant to stimulate the next idea and the next.”
So if everyone is doing it, why have Led Zeppelin received harsher criticism? Probably because as Gavin Edwards in Rolling Stone pointed out, Led Zeppelin was way too obvious.
"The Beatles swiped elements from musicians ranging from Chuck Berry to Pee Wee Crayton, but were usually careful to disguise the source. Led Zeppelin, however, took the practice further than most of their peers."
They didn't ensure that their plagiarism was "undetected." They didn't weld their theft into a "feeling which is unique." They sounded too much like their inspirations and in some cases lifted their lyrics directly from other people's songs. That was their mistake and they may never be able to shake the allegation that they are plagiarizers and musical thieves.