Duke Law's Center for the Study of the Public Domain has released an interesting article featuring the books, magazines, and movies that would have entered the public domain on January 1, 2011, if the US copyright laws weren't changed by the 1976 Copyright Act.
Copyright law, as it used to stand, was that a copyright held for 28 years and then had to be renewed for another 28 years. This caused lots of problems, due to the fact that 28 years is a long time to remember when to renew a copyright, so lots of properties were lost by their creators to the public domain. Thus, the 1976 Copyright Act, which gives copyright to the creator of the work for 70 years from the date of the creator/rights holder's death, was enacted. It simplified things and it also protected the authors and their estates. You can read about a recent copyright issue and more copyright basics here in an article that I wrote about a recent copyright infringement issue.
One of the books highlighted by Duke Law for nearly being public domain is Lord of the Flies, which would have make school teachers around the country very happy for cheaper editions. Also, remember that when copyright expires, people are free to take the work and adapt it as they see fit - in movies, in new books, in magazines. That's how Pride and Prejudice and Zombies was created - the expired copyright allowed Seth Grahame-Smith to take Jane Austen's work and alter it. And now he owns the copyright to that new, (somewhat) original work.
Also included in the list? The Fellowship of the Ring and The Two Towers, meaning those Lord of the Rings movies would have probably been put on hold until now to make, because they would have been much cheaper to do so. Dr. Seuss's children's classic Horton Hears A Who!and the pseudo-science book nearly solely responsible for the destruction of the majority of comic book publishers, Dr. Fredric Wertham's Seduction of the Innocent, also would have also been in the public domain on the 1st. The first issue of Sports Illustrated also appears, as magazines too are covered by copyright law.
Unfortunately, because of the changes in copyright law, these books won't be available to the public domain until 2050 (there's a rationale of legalese on the Duke site as to why). So, anyone planning on creating a stage version of Horton Hears a Who! will either have to wait another 40 years or pay up to Dr. Seuss's estate.