I was recently reminded of a bit of trivia I once knew but had somehow forgotten: George Romero's 1968 horror magnum opus, Night of the Living Dead is in the public domain, because its original distributor released it without a copyright notice. Consequently, anyone can now sell their own copies of the film or produce works derivative of it, including roleplaying games. I personally think that's rather nifty, but it's definitely an oddity in the modern world, where I long ago gave up hope that almost anything will ever pass into the public domain again.
This includes OD&D, which, under the terms of copyright at the time of its publication in 1974, would have entered the public domain in 2030. Those terms have since been revised multiple times, so that's an alternate reality that'll never come to pass. Fortunately, we have the Open Game License, which has given us Swords & Wizardry and Labyrinth Lord, among many other fine retro-clones and simulacra. So, whatever else I might say, my non-existent hat's off to the guys who came up with and promoted the OGL. The old school movement owes them a lot.