Here is a good list of just 10 of them (there are many more), with audio samples for each.
http://earfarm.com/features/daily-feature/monday/1820
While Led Zeppelin certainly wasn't alone in taking old blues songs and slapping their own names on them while erasing the original names, there probably has been no bigger band that stole as much without crediting the original writers.
In contrast, Eric Clapton actually made a point of searching out the old bluesmen he covered, to see if they were receiving their royalties. Many were destitute.
Legally and ethically, the songwriters should always be credited.
Music is fluid, and artists borrow from each other all the time. But in some cases, Zeppelin took whole songs.
Perhaps the most egregious cases were "Whole Lotta Love", which is essentially the same song as “You Need Love” by Willie Dixon (Dixon sued them and won), and “Babe I’m Gonna Leave You”, which was written by Anne Bredon, whose name was removed in favor of a Page arrangement credit, and was not restored until the 1990's.
I don't mention any of this to detract from the fun of listening to Led Zeppelin for its sonic pleasures; I like the whomp of their best songs as well as anyone. But they weren't very nice people, and they merit an asterisk next to their name, the way Barry Bonds' home run record deserves an asterisk for steroids.