100 years ago today, the U.S. Supreme Court handed down its decision on Bobbs-Merrill v. Strauss. Long story short, this is the decision that allows you to re-sell or give away books that you've purchased.
Half.com wouldn't exist without it. Nor could you have books in your garage sale, or sell them on ebay or at the college book store.
You wouldn't think that this would be in question, but 100 years ago, it was debatable enough to be argued before the Supreme Court. Today, it's codified in the U.S. Copyright law.
Link to John Mark Ockerbloom's article, via Boing Boing