Quoted from Billy16:
Not one of the first, they just did it best. The Yardbirds, Spencer Davis Group, Manfred Mann, Small Faces, Fleetwood Mac, Cream, Stones, Animals, etc. came before LZ playing American Blues straight ahead or rocked-up. Everyone borrowed riffs and ideas from their influences--still do. The big difference with Zeppelin--they blatantly ripped off existing tunes by bending around the music and/or slightly (or not so slightly) altering the lyrics. Not cool to claim these were then "original" compositions. It took a lawsuit or three years later to assign the writing credits to where they rightfully belonged. Others for whatever reason did not pursue legal action. Page was a master at putting together a great "new" song using other artists work. His work as a producer and session musician also gave him exposure to innumerable songs. Listen to the last recordings or live bootlegs of the Yardbirds with Page on guitar--you will hear the birth of Led Zeppelin, who did some of the same tunes as Page did with the Yardbirds (only now they were "Page" creations). Page was brilliant.
This is an argument that comes up quite often with Zeppelin, but if you read the number of disputed songs, it's fairly small in comparison to their overall library. Riffs and lyrical lines get repeated a lot throughout musical history.
But yeah, if you want to talk about "originals," you basically have to throw out all the white man rocker bands and credit the originators of the genre, which were mostly African-American blues artists. Of course, a lot of that was product of influence from swing and jazz from an era earlier.
If you want to see a good example of a "borrowed or copied" riff, go look at comparison's between Joe Satriani's "If I Could Fly," Los Enanitos Verdes "Frances Limon," and Coldplay's "Viva la Vida." Heck, just about all of Tom Petty's riffs have ended up in other songs quite prominently as well.