Answer:
   
   It is clear from the text of Genesis that the sin of eating from the
   tree of knowledge is of real significance. The mishnah (2nd cent) in
   Sanhedrin says that there are four people who didn't sin even once in
   their lives (Benjamin, Amram [Moses' father], Jesse [David's father],
   and Kilav [one of David's less famous sons]). They continue that these
   four would not have died, if it were not for that first sin.
   
   That said, Judaism does not give it the centrality that Christianity
   does. Man is not permanently tainted, nor does man face a challenge
   that means he can not redeem himself. So how does Judaism view it?
   
   Any first sin would have been "the original sin". I don't just mean
   that as a word game. What made the first sin significant is that until
   then, the desire to sin wasn't actualized. Man's whole psychology
   about sin was different; it changed from contemplating the theoretical
   to thinking about repeating what they and others had done.
   
   The way Maimonides puts it in his Guide to the Perplexed (13th Century
   CE), the pre-sin Adam knew what the goal was, his free will was to
   choose between truth and falsehood--to find the proper approach to
   that goal. R' EE Dessler puts it in Michtav meiEliyahu (early 20th
   Century CE), that until Adam ate from the fruit of the tree of
   knowledge of good and evil, the desires for good and for evil were
   external to himself. The tree of knowledge of good and evil did
   exactly what the name says--internalized insticts toward doing good
   and evil, instead of making them external realities.
   
   Perhaps these two opinions are different perspectives on the same
   thing. As external realities, if a person would want to do good, the
   challenge would be in figuring out what good is. Now, however, you
   have an instinct, a spiritual ear that hears the calling of G-d, the
   challenge is to overcome your other urges. But we believe that man is
   in perfect balance even after the sin. The domain over which he
   chooses was changed, but man is still fully free willed, poised
   between each side. He is not inherently evil.