Here is a chronological comprehensive (if I've missed anything please feel free to point it out) list of songs Led Zeppelin supposedly ripped off. Some are 100% factual and I will call a pig a pig everytime, but other charges are baseless imo and I will also point those out, as well as seek input from other listeners. It is also important to remember that there was no internet back then. In some cases they simply might not have known who to give credit to as the information was much much harder to track down and verify.
Babe I'm Gonna Leave You - Written by Anne Bredon, but the version LZ lifted was Joan Baez's from her Live album in 1963. They did give proper credit to Anne Bredon, so no ripoff here, just a cover. Here is a clip of Baez doing the song:
You Shook Me - Purely a cover. Written by and properly credited to Willie Dixon.
Dazed & Confused - Pure Theft. Jake Holmes' original song is titled "I'm Confused", and Jake just happened to open for the Yardbirds during Jimmy's time with them. The lyrics have been changed, but the music is verbatum albeit much heavier. In fact, Jimmy had already stolen this song while still with the Yardbirds and did the song in concert several times before forming LZ. Interesting side note is Jake was in an entertainment "trio" which included Joan Rivers. He later went on to have much success writing many tv jingles, including "Gillette the best a man can get"; "Zack the Leggo maniac, The Army's "Be all that you can be" and perhaps the most famous "I'm" a Pepper" for Dr. Pepper.
Your Time is Gonna Come - The first questionable one imo. The charge is that Jimmy stole the 7 note riff (before the chorus) from Traffic's "Dear Mr. Fantasy", and that the melody and chord progressions are a rip off of Blind Faith's "Can't Find My Way Home". Now while I have to concede the 7 note riff part of the claim, I have a hard time believing the second part of it. It is definitely similar, but Led Zeppelin I was released in January of 1969, Blind Faith's record wasn't released until August of 1969. It is true that Winwood and Clapton were jamming together since the Summer of 1968, but it seems unlikely that Jimmy could have heard that song before the release of LZ I.
White Summer/Black Mountain Side - Pure theft. This is from a traditional British folk song titled "She Moved Through the Fair", and the version appearing on Zeppelin I is lifted from Bert Jansch's interpretation titled "Black Waterside".
Communication Breakdown - The charge is that this is a rip off of Eddie Cochrane's "Nervous Breakdown". I don't see it at all. The only similarity I can see is the word "Breakdown" in the title. Maybe you can see something I do not...
I Can't Quit You Baby - Pure cover. Willie Dixon song and properly credited.
How Many More Times - This one is a bit tricky. The claim is LZ ripped off a Howlin' Wolf song titled "How Many More Years". I can't see this one either. The songs do not have anything in common to my ears, and I think it is a baseless claim. However, at the 6:15 mark Zeppelin definitely goes into "The Hunter", which is a song written by Booker T and the MGs, and first recorded by Albert King. There is no denying they are playing The Hunter from 6:15-7:05. So basically :50 seconds of a 8 minute+ tune is definitely "ripped off". Does that make the whole song a rip off? I can't see how. Here's the Wolf song, maybe you can hear something similar between them? And then we have Albert King's the Hunter, no mistaking that song near the end of Zep's tune as mentioned above: