Sometimes I think people exaggerate in trying to accuse authors of plagiarism. I cannot say anything about Eragon, having not read the book nor seen the movie. J.R.R. Tolkien however took uncountable inspirations from old legends, Atlantis, Beowulf, ancient greek and nordic legends... Tolkien's writings are full of this, but it doesn't diminish his works the least in my opinion. Tolkien himself, when asked about his sources, somewhat annoyed responded that one should enjoy the soup rather than asking for the bones of the ox it was made of.
Of course I see that there are books which overstep a certain border to the degree where they really change way too few elements from a known story to be perceived as individual works. However I also find that there is hardly any new book, in the genre of fantasy in particular, which is not being accused of being plagiarized.